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Whether Java SE is the same as JDK? [duplicate]

This question already has answers here:
Java JDK, SDK, SE?
(4 answers)
Closed 5 months ago.
Me has been fighting with internet for a while for that single answer. Everything else now seems standing on it's place after this trip.
When I ask "What's Java SE" they say "Java SE is set of core Java classes ...... platform ... develop desktop apps .... contains core libraries and APIs ..... Java EE is a spec set to be implemented ...... Java EE is built on top of the Java EE". Sometimes they mention javac and JVM, sometimes they don't. What they do never (almost) is mention JDK.
When I ask "What's JDK" they say "JDK is set of stuff that lets us build apps on Java including javac, JVM etc." and never (almost) mention Java SE.
When I ask "Difference between Java SE and JDK" they give me that page and nothing more to think about. What does this mean: "Java SE product name for JDK"? Can't clearly understang right now.
Lack of terminology presicion and experiece for me? Possibly yes. I think would be able to answer this question myself if I were more experienced.
The questions:
"Whether (Java SE) == (JDK) or not?" (pls with wide explanation and terminology clarifications when needed).
Is Java SE a spec set like Java EE or not?
Can I find precise definitions somewhere to look at?
Sorry for a big writing. It proves that I would really love to put an end to this.

“Java SE” is three things:
An abbreviation of “Java Standard Edition”
A trademark owned by Oracle Corp.
A set of specifications published by Oracle Corp.
Anybody in the world is free to write software that implements the Java SE specifications. In the past, several companies have done so.
However, anybody in the world is not free to use the trademark of Java. To use that trademark, a person or company is required to come to terms with Oracle in a licensing deal. Presumably that deal would require payment.
Long story short, and over-simplified for brevity… Some of the companies that previously wrote their own implementations of Java SE decided to join forces, combine their codebases, and publish the whole thing as open-source. These companies included Oracle, IBM, and Apple. Thus was born the OpenJDK project (Wikipedia). Since then, other companies and individuals have joined the project.
The OpenJDK name is a trademark owned by Oracle. However, Oracle has published permission allowing others to use the term with no payment required.
The OpenJDK project published source code to an implementation of the Java SE specifications. The project does not publish built binaries or installers.
Several vendors do publish built binaries or installers for implementations of the Java SE specs. Nearly all of these implementations rely largely, if not entirely, in the OpenJDK source code.
These binaries/installers vendors include, in no particular order:
Azul Systems
SAP
Amazon
Microsoft
IBM
Red Hat
Oracle
Adoptium
BellSoft
… and more.
Most of those built binary or installers products use OpenJDK in their name or in their descriptions, as allowed by Oracle. A few of those products may use the term Java or Java SE with explicit permission from Oracle.
There are other editions of Java technology:
Java Card (for smart cards, badges, etc.)
Java ME, or Micro Edition (for constrained devices)
Java EE, or Enterprise Edition (a wide collection of specifications that run on top of an implementation of the Java SE specs, aimed at developers making software for enterprise oriented users)
The first two, Java Card & Java ME are owned by Oracle. The third, Java EE, was donated by Oracle Corp to the Eclipse Foundation were it is now known as Jakarta EE. Jakarta EE is a collection of specifications, with various vendors publishing implementations.
The term “JDK” refers to Java Development Kit. This kit includes an implementation of the Java SE specs, plus tools to write Java apps. These tools include a compiler.
The term “JRE” refers to Java Runtime Environment. This is a JDK minus some of the software development tools.
The term “JVM” refers to Java Virtual Machine. A JVM is a piece of software that enables a Java-based app to execute at runtime on a particular kind of host computer and operating system. Java apps are platform-agnostic, in other words, are not “native”. So a Java app needs a particular JVM to know how to run on a Mac, on a PC, or on a mainframe.
Every JDK and every JRE contains a JVM.

1 . Yes, you can think of Java SE same as JDK.
Java SE is the 'base' platform that defines the Java language specification (JLS), the Java Virtual Machine Specification (JVMS), Standard library APIs, byte-code format, compilation tools, and the class loader.
Now, this is all concepts and specifications and community proposals (via JCP) until implemented into a set of software tools. The set of software and tools that implements all of the above is called JDK.
JDK is essentially a Software Development Kit (SDK). They simply decided to name it JDK.
2 . Yes, Java SE is basically a set of specs. An implementation must follow the spec and get it reviewed & approved by Oracle before they can call their implementation Java.
In the case of Java EE the specifications deal with libraries & APIs most of the time. In case of Java SE the specs also care about language and VM details. You take Java SE (the standard JDK) + add some libraries and now you have Java EE. Oracle refers to this as Java EE SDK though.
3 . Unless your life depends on it. Just understand the relationship between Java SE, Java EE, JDK, SDK, and you are good. Just connect the dots and be done with it.

I think your confusion stems from what Java is. Originally, most languages were like C, where you write code, and it gets compiled to a specific computer architecture/ISA (x86, ARM, whatever).
Java "changed the game" when it came on the market, because it was:
Free (many langs at the time were proprietary)
Enforced object oriented programming style (which all the cool kids were doing at the time)
It added the JVM. The JVM is installed on top of the ISA, it was a piece of middleware that meant that compiled Java code could "run anywhere" (in practice, this isn't strictly true, but it's aside the point)
Anyways, if you just need to run Java programs, you can just install the JVM/JRE (of which the JVM is a component). If you actually need to develop Java programs, you need one of these "editions," which supports more or less features, depending on what you are doing with it. If you're a hobbyist programmer, SE is fine. SE, EE, and ME all include the JRE, JVM and JDK.

For a start here is the language specification (what's the meaning of for) itself and the JVM (Java Virtual Machine) specification. A JVM knows how to execute the code in the end. (see https://docs.oracle.com/javase/specs/)
"Java SE" is the "Standard Edition" as opposed to "Java EE" which is the "Enterprise Edition"
Enterprises create webservers so Java EE specifies many things needed for running Java based Webservers like how to respond to web requests. There also additional libraries and webservers that adhere to that Java EE spec but they are still written in Java, i.e. what you do with plain Java (SE). See https://www.oracle.com/java/technologies/javaee-8-sdk-downloads.html
To run any kind of Java program you you'll need a JRE (Java Runtime Environment) which is in a way part of Java SE because it bundles all the core functionality of Java. The core libraries (like the implementation of the String class) as well as a JVM implementation (Oracle, OpenJDK, ...). Even an EE webserver would still require a JRE to run.
If you want to develop a Java program, you will also need a JDK (Java Development Kit) which is essentially a JRE plus extra tools, the compiler for example. The compiler takes .java files and create .class files which is what the JVM needs.
It's also in a way part JavaSE but I wouldn't says JavaSE = JDK.
Java SE and just "Java" is probably the same, the other terms all go into different aspects of Java.

How to migrate java application form oracle jdk to openjdk?

we have one java application which is deployed in jboss application server and using java 1.8.0_202 version, now we want to change it to openjdk.
how can i change?
is there any code changes required?
please help me complete steps to be performed?
Thanks in advance..

No changes required to your codebase when switching between implementations of Java 8. Java is defined by a set of specifications along with JSRs and JEPs. Any implementation that complies with those specs will run your JBoss app server. Your existing WAR or EAR file will run as-is with no changes needed.
Understand that the OpenJDK project provides an implementation of Java as source code. The OpenJDK project does not provide binary builds nor installers. For builds and installers, you must locate a vendor. You have a choice of several, some free-of-cost and some that require a fee.
Here is a flowchart I made to help you choose a vendor.
This chart is aimed at Java 11, but many of these vendors also supply Java 8. Some, such as Azul Systems, have even back-ported features to Java 8 such as Flight Recorder.

Which free version of Java can I use for production environments and or commercial purposes?

While I was in a dockerization project in my current job, I got this doubt: Which Java can I use?
This doubt is due to :
So after some researches I got these conclusions:
Since the Java 8 update at April 16, 2019 8u221, all versions and updates for (Java 8, 9, 10, 11 and 13) has no cost just for personal use and development purposes. Any other use, needs a Commercial License
Legacy versions prior to 7, does not have and will not have any update. Maybe a sales contact could be a solution if an update in these versions are required for Legacy Systems Support.
If I want to use Java 8 oracle version for commercials purposes and FREE, I need to use a previous version of April 16, 2019 8u221 update
Questions
According to my conclusions:
If my development strictly needs or was developed with JDK 8, Can I only use Oracle Java SE 8 JDK 8u202 and earlier versions for FREE and commercial purposes accepting issues and security problems?
If I can change my development and I want still FREE, and I want to have improvements and security updates, I must use one of these java open implementations?
References
End of Public Updates for Oracle JDK 8
The Oracle JDK License has changed for releases starting April 16, 2019.
Official List of updates and release versions
Java official supported versions
Oracle Java 8 download alert
Oracle Java 8 unsupported but free version
Java 4 legacy official download page
Java 5 legacy official download page
Java 6 legacy official download page
Java 7 legacy official download page

Update 2021-09
For versions 8 through 16, Oracle required a fee if their own Oracle JDK product was used in production, but not for dev, test, and training usages.
For Java 17, the Oracle JDK product is available under a new
No-Fee Terms and Conditions license, discussed on the Oracle
company blog.
On my first reading, it appears this new license makes production use free-of-cost (along with dev, test, and training usages), except for products sold for a fee while bundling the Oracle JDK product. But I am not an attorney, so read the terms yourself and consult legal advice as needed.
Keep in mind that many other vendors continue to provide implementations of the Java specs, as shown in the flowchart below. Some of these vendors sell support plans, either optionally or as a requirement for use of their product. Never assume, always read the detailed requirements for any distribution you obtain.
Another 2021 update: Add Microsoft to the list of vendors seen below.
Several vendors offer a choice of Java implementations
The Answer by Speakjava is correct and informative.
In addition, here is a flowchart I made to guide you in choosing a vendor for a Java implementation.
Or another way to view this: Your particular motivations or situation.

Oracle made a number of significant changes to how the Java platform is developed, distributed and updated at the same time as the launch of JDK 9.
The key facts to answer your questions are these:
Oracle was using the Oracle Binary Code License (OBCL) for Java. This had field of use restrictions (you needed a commercial license for embedded use) but granted free use of the Oracle JDK for general purpose development and deployment.
As of JDK 11, the license changed to the Oracle Technology Network License Agreement (OTNLA), which requires you to purchase a Java SE Subscription for use in commercial production. This license has also been applied to updates of JDK 8 since April (update 211/212).
As of JDK 11, Oracle eliminated all functional differences between the Oracle JDK and a binary built purely from the OpenJDK source code. The important things no longer in the Oracle JDK 11 are the browser plugin, Java Web Start and JavaFX.
You can continue to use any public Oracle binaries of JDK 7 (and earlier) in production without the need for a Java SE subscription, as the license is still the OBCL (it has not been changed retrospectively). You can also, as you say, use JDK 8 up to update 202 under the OBCL without cost.
There are several options for continuing to use JDK 8 with updates for free. One of those is Zulu Community from Azul (who I work for). This is a free, TCK-tested binary with a couple of additional backported features (TLS 1.3 and Java Flight Recorder). Since this passes the TCK, from a functional perspective, it is identical to the Oracle JDK. Unless you are using Applets or Web Start your applications will work in the same way as with the Oracle JDK.
If you want to move to JDK 11 (the current long-term support release, as defined by Oracle but followed by all other JDK providers), then you will need to choose one of the free distributions (like Zulu Community).
It is highly recommended to ensure your JDK is up to date and contains all relevant security patches. Continuing to use an older version could expose you to significant security exploits.

Summary
If you need to have java in your production servers for free, your have two options
#1 OpenJdk
The OpenJDK is the open source reference implementation of the Java SE Specification, but it is only the source code https://github.com/openjdk
Binary distributions are provided by different vendors for a number of supported platforms.
One of them is https://jdk.java.net/java-se-ri/19 which is maintained by Oracle:
Source: https://whichjdk.com
#2 Customized by Third Parties
Amazon Corretto
https://docs.aws.amazon.com/corretto/latest/corretto-11-ug/downloads-list.html
Zulu
https://www.azul.com/downloads/zulu-community/?architecture=x86-64-bit&package=jdk
Licences OTN vs BCL
Oracle JDK 8 (aka 1.8) no longer uses BCL (Binary Code License). From April 16, 2019, Oracle JDK 8 uses the OTN (Oracle Technology Network) license, which requires you to create an Oracle account to download JDK 8 and payment!!
BCL = Oracle Binary Code License
You can use it, but you can't modify it
You agree not to sue Oracle if anything goes wrong
You can redistribute/publish it (so that you can sell products with Java embedded), but if you do, you agree to indemnify Oracle; so if someone sues you, you can't drag Oracle into it.
It's really just there to protect Oracle's intellectual property and to shield them from being sued when bugs are found.
source: https://www.quora.com/In-short-what-does-the-Oracle-Binary-Code-License-Agreement-for-Java-SE-actually-say-or-prohibit/answer/Jon-Harley
As a summary: FREE with risks and without any fault of Oracle
OTN = Oracle Technology Network License
As a summary: Opposite to BCL and FREE just for development in your laptop. For enterprises, you must PAY
OpenJDK and Oracle JDK
Both OpenJDK and Oracle JDK are created and maintained currently by Oracle only.
OpenJDK and Oracle JDK are implementations of the same Java specification passed the TCK (Java Technology Certification Kit).
Most of the vendors of JDK are written on top of OpenJDK by doing a few tweaks to [mostly to replace licensed proprietary parts / replace with more high-performance items that only work on specific OS] components without breaking the TCK compatibility.
Source: Differences between Oracle JDK and OpenJDK
Free official options
Here I will list and keep updated the official links, ready to download the most used java versions
Openjdk 19
https://jdk.java.net/java-se-ri/19
Openjdk 17
https://jdk.java.net/java-se-ri/17
Openjdk 11
https://jdk.java.net/java-se-ri/11
apt-get update
apt-get install openjdk-11-jdk
Oracle Java 8 update 202
Just Java SE 8 JDK 8u202 and earlier versions are free for development and production deployment. You can download it from:
https://www.oracle.com/java/technologies/javase/javase8-archive-downloads.html
Openjdk 8 (Deprecated)
OpenJDK is a ORACLE initiative. More details here: https://adoptopenjdk.net/
compressed mode
You can download the latest v8 release from here. Latest version:
https://github.com/AdoptOpenJDK/openjdk8-upstream-binaries/releases/download/jdk8u292-b10/OpenJDK8U-jdk_x64_windows_8u292b10.zip
https://github.com/AdoptOpenJDK/openjdk8-upstream-binaries/releases/download/jdk8u292-b10/OpenJDK8U-jdk_x64_linux_8u292b10.tar.gz
I can't find the version for osx :(
Follow this to download using curl
install mode
apt-get install openjdk-8-jre (just run apps)
apt-get install openjdk-8-jdk (develop and run)
From https://openjdk.java.net/install/
Oracle Java 1.4, 5, 6 and 7 (Deprecated)
Oracle does not show any message related to license changes for Java 1.4, 5, 6 and 7 downloads. So we can use them for development and production deployment, accepting issues and security problems because these versions are so ancient!!
Oracle downloads:
https://www.oracle.com/java/technologies/java-archive-javase-v14-downloads.html
https://www.oracle.com/java/technologies/java-archive-javase5-downloads.html
https://www.oracle.com/java/technologies/javase-java-archive-javase6-downloads.html
https://www.oracle.com/technetwork/java/javase/downloads/java-archive-downloads-javase7-521261.html
Legacy versions prior to 7, does not have and will not have any update. Maybe a sales contact could be a solution if your have a Legacy Systems running over this java old versions.
Free From Trusted Third Parties
Zulu Community (Java 6,7,8,11,13,14,15)
https://www.azul.com/downloads/zulu-community/?architecture=x86-64-bit&package=jdk
Amazon Corretto (Java 11)
https://docs.aws.amazon.com/corretto/latest/corretto-11-ug/downloads-list.html
Eclipse OpenJ9
https://www.eclipse.org/openj9/
More third parties
The following implementations, listed in alphabetical order, are open source and free to use:
AdoptOpenJDK
Azul Zulu
Bck2Brwsr
CACAO
Codename One
DoppioJVM
GraalVM CE
HaikuVM
HotSpot
Jamiga
JamVM
Jelatine JVM
Jikes RVM (Jikes Research Virtual Machine)
JVM.go
leJOS
Maxine
Multi-OS Engine
RopeVM
uJVM
NON-FREE options
You should pay for these versions but in return you will have a lot of features suported by Oracle or another third parties
Oracle Java
https://www.oracle.com/java/technologies/downloads/
Java 19
https://www.oracle.com/java/technologies/downloads/#java19
Java 17
https://www.oracle.com/java/technologies/downloads/#java17
Java 11
https://www.oracle.com/java/technologies/downloads/#java11
Oracle Java 8 update 221
Since the java 8 update at April 16, 2019 8u221, all versions and updates for (java 8,9,10,11,14) has no cost just for personal use and development purposes. Any other use, needs a Commercial License.
https://www.oracle.com/java/technologies/javase/javase-jdk8-downloads.html latest update 251
https://www.oracle.com/java/technologies/javase/javase8u211-later-archive-downloads.html previous updates (241,231,221,212,211)
Source: https://www.baeldung.com/oracle-jdk-vs-openjdk
Proprietary Implementations
There are also other private or commercial implementations:
Azul Zing JVM
CEE-J
Excelsior JET (Discontinued)
GraalVM EE
Imsys AB
JamaicaVM (aicas)
JBlend (Aplix)
MicroJvm (IS2T – Industrial Smart Software Technology)
OJVM
PTC Perc
SAP JVM
Waratek CloudVM for Java
Source: https://www.baeldung.com/oracle-jdk-vs-openjdk
Notes
Oracle Java 9 and 10 has reached end of support.
https://stackoverflow.com/a/50333498/3957754
More References
Differences between Oracle JDK and OpenJDK
https://whichjdk.com/
https://www.openlogic.com/blog/java-experts-openjdk-vs-oracle-jdk
https://www.oracle.com/technetwork/java/javase/overview/faqs-jsp-136696.html
https://www.oracle.com/downloads/licenses/javase-license1.html
https://openjdk.java.net/projects/jdk8/
https://www.oracle.com/java/technologies/javase/8u-relnotes.html
https://gist.github.com/jrichardsz/83db09163ca9a0db4c9cd4f91cbf0598/
https://jdk.java.net/archive

Starting JDK 17 (September 2021)
Oracle JDK 17 and later are provided under a free-to-use license that lasts until one year after the subsequent LTS release. After that time, the Oracle JDK becomes under OTN license.
Here's a related FAQs page that explains more, but here's a summary extracted from that page:
Oracle OpenJDK releases are under GNU GPL v2 with the Classpath Exception (permits free production use).
Oracle JDK 17 and later is available under the Oracle No-Fee Terms and Conditions License which permits free use for all users.
Previous Oracle JDK releases remain under the OTN License Agreement for Java SE.
"Oracle will use the NTFC for JDK 17 and later releases. LTS releases, such as JDK 17, will receive updates under this license for one year after the release of the subsequent LTS. After the free use license period, Oracle intends to use the OTN License, the same currently used for Java 8 and 11 LTS releases, for subsequent updates. Non-LTS releases such JDK 18 will be available for their entire planned six months support life under the NFTC."
Sources:
https://blogs.oracle.com/java/post/free-java-license
https://www.oracle.com/uk/java/technologies/javase/jdk-faqs.html

Now,You can use JDK/Java 17, in JDK/java7 download page(https://www.oracle.com/java/technologies/downloads/) ,you can get this comment:
Java 17 LTS is the latest long-term support release for the Java SE platform. JDK 17 binaries are free to use in production and free to redistribute, at no cost, under the Oracle No-Fee Terms and Conditions License.
JDK 17 will receive updates under these terms, until at least September 2024.

What is the reason to use OpenJDK?

I somehow understand the differences between Oracle JDK and Open JDK.
But I can't find reasons to use Open JDK.
Because Oracle JDK already has everything one would need and sometimes faster than Open JDK.
In which cases should someone use Open JDK?
Is it just for legality issues?
If yes, then what are issues?
That because I have been still develop all my projects using Oracle JDK

Other Answers are correct. Here is some other information as well, including vital changes as of early 2021.
Source-code versus Binaries/installers
understand the differences between Oracle JDK and Open JDK.
To sum it up: source-code versus binaries/installers.
OpenJDK is an open-source project, implementing the Java Specifications, JSRs, and JEPs that define the Java platform. This project publishes only source-code, not binaries or installers to get Java running on your computer (except for some early-access builds). Oracle is the principal participant in the OpenJDK project, alongside IBM & Red Hat, Apple, SAP, Azul Systems, Microsoft, and others.
Many companies provide distributions of OpenJDK for installation, as binaries or installers. Oracle is one such company.
Oracle actually provides two such products:
Oracle JDKA commercial product, with paid support plans. (Free-of-cost only for development & testing, not deployment. Read their terms.)
jdk.java.netA build of the OpenJDK source-code, freely available, unsupported.
Oracle has declared their intention to keep their commercial product Oracle JDK at feature-parity with OpenJDK. Oracle even went so far as to open-source and make available at no cost their formerly commercial tools Mission Control and Flight Recorder, both now a part of OpenJDK. So there is nearly no practical difference. One significant difference is that the Oracle company reserves the right to rapidly supply their paid support customer base with urgent patches not currently found in OpenJDK. Ditto for other vendors selling support.
[And for the record, Oracle offers a third implementation of Java, GraalVM, which is a very specialized product.]
Here is a flowchart graphic I made to help guide you in selecting a provider of a Java implementation.
And here is a list of motivations you might consider in choosing a distribution.

November 2017: At the time this question was asked, the reasons for choosing between OpenJDK and Oracle JDK include:
Availability of builds / installers for specific platforms. For example, last time I checked there were no Oracle JDK distros in ".deb" format for Debian / Ubuntu. And no OpenJDK builds for Windows.
Oracle JDK has some commercial features that can be enabled by people who have paid. They are not present on OpenJDK.
Oracle JDK has an annoying click-through license. This is a pain if you are trying to automate deployment; e.g. to cloud VMs. By contrast, OpenJDK on Linux (at least) can be installed and updated using the platform's package manager.
There may be issues if you wanted commercial support for OpenJDK.
There were no substantive license differences between Oracle JDK and OpenJDK if you just plan to use the software. (There are some constraints on binary redistribution for Oracle JDK, but they are about shipping non-standard or stripped-down versions of the Java platform.)
In the future, some of these differences will go away, according to this blog page:
Faster and Easier Use and Redistribution of Java SE
Update in December 2018:
As of January 2019, Oracle JDK Java 8 will be EOL for commercial uses. (But free support will continue until January 2020 for personal use.)
1
Java 9 and Java 10 are EOL.
Commercial uses of Oracle JDK Java 11 and later require a support contract or a subscription from Oracle. OpenJDK Java builds do not have this requirement.
Oracle do not provide support for OpenJDK Java builds, but support is available from 3rd parties. Third parties will also port Oracle security fixes to OpenJDK builds.
Further reading:
Java Is Still Free
OpenJDK Life Cycle and Support Policy (RedHat)
The future of Java and OpenJDK updates without Oracle support
What has changed in Java Release and Update Availability? (Azul Systems)
AdoptOpenJDK Support
1
EOL stands for end-of-life.
An end-of-life (EOL) product is a product that does not receive continuing support, either because existing marketing, support and other processes are terminated, or it is at the end of its useful life.

OpenJDK comes tightly integrated with the distribution update process. For example, on Fedora, this means that you simply can run
dnf update
without activating third-party repositories, and you will get the latest OpenJDK version with all relevant fixes.
OpenJDK is tested by the distribution vendor on this particular distribution. Oracle has different reference platforms for testing of OpenJDK. In the case of Fedora, software written in Java is tested with the OpenJDK version provided by the distribution, while the combination of that software with Oracle JDK receives much less testing.
Oracle also bundles their Java downloads with additional software such as browser toolbars, performance measurement and monitoring tools.

For those, who read this in 2019.
Java SE 8 is the last Java SE release targeting Desktop deployment (eg, Java Web Start and a stand-alone system JRE). Java SE 8 will be end of public updates in January 2019, after which commercial users need to either transition to other technologies, or purchase a Java SE Subscription.
So, another reason to use OpenJDK (in addition to what Stephen C said): it's free of charge for all purposes.

sun's application server?

in a tutorial they asked me to install the application server which could be downloaded from their site.
what do they mean with application server?
the link they provided: http://java.sun.com/javaee/downloads/index.jsp
i mean come on sun, all these names. jdk then j2ee..and when i wanna download j2se it says jdk6. r they insane? a lot of people are wondering what is what...they just dont know how to name things.
however, back to the question. so with application they mean java ee server?
i've got a mac and people say java is already installed. what is installed exactly? the j2ee or j2se? do i have do download j2ee? Java is just killing me...
EDIT: read something about that the application server is a name for ee server. and there are many ee servers like tomcat, jetty and glassfish. and i know these sometimes are called servlet containers.
so that makes application server = ee server = servlet container?
and jdk = j2se? so j2ee != jdk?

Sun's Application Server has been superseded by the community app server, GlassFish.
The JavaEE download page only has two JavaEE6 download links and both contain versions GlassFish v3. Chances are you only need the web profile, but download the full version if you aren't sure.
Note that GlassFish is just the reference Application Server. You also have other Application servers, such as Apache Geronimo and Oracle Weblogic... as well as ones that are just servlet containers, like Apache Tomcat and Jetty. These types should correspond to GlassFish's normal and web profiles respectively.
These other servers only require a JRE to run applications, or the JDK to develop for them.
Side Note: GlassFish v3 is the only JavaEE 6-compliant server... JavaEE 6 is brand new. The others should be JavaEE 5 compliant.
Also, JavaEE is the new name for J2EE, as sun tries to move away from the Java 2 name.

What do they mean with application server? The link they provided: http://java.sun.com/javaee/downloads/index.jsp
An application server is a component-based middleware used in server centric N-tier architecture. It manages the life-cycle of components deployed on it, it provides services for state maintenance, data access (with pooling of resources), security, clustering and fail-over.
AFAIK, one of the first application server (as just defined above) was ATG Dynamo. Other proprietary application servers include BroadVision, ColdFusion, etc. But none of them really survived to the advent of Java application servers (understand application servers based on the standards defined by Java EE and the Java language).
I mean come on sun, all these names. (...)
Wikipedia does a pretty good job at defining what Java SE is:
Java Platform, Standard Edition or Java SE is a widely used platform for programming in the Java language. It is the Java Platform used to deploy portable applications for general use. In practical terms, Java SE consists of a virtual machine, which must be used to run Java programs, together with a set of libraries (or "packages") needed to allow the use of file systems, networks, graphical interfaces, and so on, from within those programs.
The JRE (Java Runtime Environment) provides the virtual machine and the set of libraries i.e. everything you need to run Java software. The JDK (Java Development Kit) provides a JRE plus a compiler (javac) and some other tools i.e. everything you need to run and develop Java software.
Java EE is a specification (more precisely, a set of specifications) built on top of the libraries provided by Java SE and is more server-side oriented. Implementations of this specification are provided by Java application servers: GlassFish is Sun's implementation, JBoss is RedHat's implementation, WebLogic is BEA Oracle's implementation, WebSphere is IBM's implementation, etc.
Regarding the versions and nomenclature, yes, Sun is crazy. It was an horrible mistake to introduce this "Java 2 Platform, Standard Edition" naming when the version 1.2 came out. I can imagine how confusing this is. But, again, Wikipedia does a great job at clarifying this in the section Nomenclature, standards and specifications:
Java SE was known as Java 2 Platform, Standard Edition or J2SE from version 1.2 until version 1.5. The "SE" is used to distinguish the base platform from Java EE and Java ME. The "2" was originally intended to emphasize the major changes introduced in version 1.2, but was removed in version 1.6. The naming convention has been changed several times over the Java version history. (...)
This should make things more clear.
(...) however, back to the question. so with application server they mean java ee server?
Yes, this is what they mean or, more precisely, they mean Sun GlassFish Enterprise Server (previously named Sun Java System Application Server). Source: http://developers.sun.com/appserver/.
I've got a mac and people say java is already installed. what is installed exactly? the j2ee or j2se? do i have do download j2ee? Java is just killing me...
A JRE or a JDK (if javac available, it's a JDK), so only the Java SE part. And actually, if you are still following me, you don't download Java EE, you download something providing an implementation of it (i.e. an application server).
(...) so that makes application server = ee server = servlet container?
No. To simplify, a Java EE server = Servlet container + EJB Container. Some server are only Servlet container (like Tomcat, Jetty), they don't provide the EJB container part and thus don't fully implement the Java EE specification and can't be considered as full Java EE servers.
and jdk = j2se? so j2ee != jdk?
I hope I covered this with my answer.

In order to run you Java EE applications you need an application server.
The link you provided is to download GlassFish that is one application server.
There are others application servers like Apache Tomcat, Jboss.
JDK stands for Java Development Kit
You need this to develop Java applications.
JRE stands for Java Runtime Environment
You need this to run Java applications.

GlassFish with the Java EE SDK (provided on the link you gave) is what you need. Java EE server = application server.
What is installed on your Mac by default is probably only a JRE (runtime environment) and not a JDK (development kit). Which means, you can run Java apps but not develop. By downloading GlassFish with the Java EE SDK, you'll get the full-blown SDK you need and a server to deploy on.
Hope that helps.