What is the difference between JDK and JRE?

The JRE is the Java RunTime Environment that is a plug-in needed for running java programs. The JRE is an implementation of the Java Virtual Machine which actually executes Java programs.
The JDK is the Java Development Kit for Java application developers. The JDK is bundle of software which contains one (or more) JRE's along with the various development tools like the Java source compilers, bundling and deployment tools, debuggers, development libraries, etc.