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 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 Javaplatform. 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 Javaplatform.
Source Code
(In src.zip) Java programming language source files for all classes that make up the Java core API (that is, sources 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 file, 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