From Sun Microsystems:
Java Development Kit 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.
What's new in this version:
Version 7 Update 21 includes blacklisted jars and certificates, changes to Java control panel's security settings, changes to security dialogs, changes to RMI server JRE, and JDK for Linux on ARM.