Java Development Kit (also known as JDK) contains the software and tools that you need to compile, debug, and run applets and applications that you've written using the Java programming language. JDK has as its primary components a collection of programming tools, including javac, jar, and the archiver, which packages related class libraries into a single JAR file. This tool also helps manage JAR files, javadoc - the documentation generator, which automatically generates documentation from source code comments, jdb - the debugger, jps - the process status tool, which displays process information for current Java processes, javap - the class file disassembler, and so many other components.
The JDK also comes with a complete Java Runtime Environment, usually called a private runtime. It consists of a Java Virtual Machine and all of the class libraries present in the production environment, as well as additional libraries only useful to developers, and such as the internationalization libraries and the IDL libraries.
Contents of the JDK:
Development Tools
(In the bin/ subdirectory) Tools and utilities that will help you develop, execute, debug, and document programs are written in the JavaTM programming language.
Runtime Environment
(In the jre/ subdirectory) An implementation of the Java Runtime Environment (JRE) for use by the JDK. The JRE includes a Java Virtual Machine (JVM), class libraries, and other files that support the execution of programs written in the Java programming language.
Additional Libraries
(In the lib/ subdirectory) Additional class libraries and support files required by the development tools.
Demo Applets and Applications
(In the demo/ subdirectory) Examples, with source code, of programming for the Java platform. These include examples that use Swing and other Java Foundation Classes, and the Java Platform Debugger Architecture.
Sample Code
(In the sample subdirectory) Samples, with source code, of programming for certain Java API's.
C header Files
(In the include/ subdirectory) Header files that support native-code programming using the Java Native Interface, the JVM Tool Interface, and other functionality of the Java platform.
Source Code
(In src.zip) Java programming language source files for all classes that make up the Java core API (that is, source files for the java.*, javax.* and some org.* packages, but not for com.sun.* packages). This source code is provided for informational purposes only, to help developers learn and use the Java programming language. These files do not include platform-specific implementation code and cannot be used to rebuild the class libraries. To extract these files, use any common zip utility. Or, you may use the Jar utility in the JDK's bin/ directory: jar xvf src.zip.
Also Available: Download Java Development Kit for Mac

Java Runtime Environment (JRE) allows you to play online games, chat with people around the world, calculate your mortgage interest, and view images in 3D, just to name a few. It's also integral to the intranet applications and other e-business solutions that are the foundation of corporate computing.
It provides the libraries, the Java Virtual Machine, and other components to run applets and applications written in the Java programming language. In addition, two key deployment technologies are part of the JRE: Java Plug-in, which Enables Applets to Run in Popular Browsers; and Web Start, which deploys standalone applications over a network. Many cross-platform applications also require Java to operate properly.
Itis a programming language and computing platform first released by Sun Microsystems in 1995. There are lots of applications and websites that will not work unless you have Java installed, and more are created every day. The program is fast, secure, and reliable. From laptops to datacenters, game consoles to scientific supercomputers, cell phones to the Internet, Java is everywhere! Itis at the heart of the digital lifestyle. It's the platform for launching careers, exploring human-to-digital interfaces, architecting the world's best applications, and unlocking innovation everywhere—from garages to global organizations.
Why should I upgrade to the latest version?
The latest version contains important enhancements to improve the performance, stability, and security of the applications that run on your machine. Installing this free update will ensure that your applications continue to run safely and efficiently.
What will I get when I download software?
The Java Runtime Environment (JRE) is what you get when you download software. The JRE consists of the Java Virtual Machine (JVM), Java platform core classes, and supporting Java platform libraries. The JRE is the runtime portion of the software, which is all you need to run it in your Web browser.
What is Java Plug-in software?
The program is a component of the (JRE). The JRE allows applets written in the programming language to run inside various web browsers. The Plug-in software is not a standalone program and cannot be installed separately.
I have heard the terms Virtual Machine and JVM. Is this software?
The Virtual Machine is only one aspect of software that is involved in web interaction. The Virtual Machine is built right into your software download and helps run Java apps.
Note: When your installation completes, you may need to restart your browser (close all browser windows and re-open) to enable the installation.
Also Available: Download Java Runtime Environment for Mac