Hi :)
Java 6 Update 20.

Java software allows you to run applications called "applets" that are written in the Java programming language. These applets allow you to have a much richer experience online than simply interacting with static HTML pages.

Java Plug-in technology, included as part of the Java 2 Runtime Environment, Standard Edition (JRE), establishes a connection between popular browsers and the Java platform. Java allows applications to be downloaded over a network and run within a guarded sandbox. Security restrictions are easily imposed on the sandbox.

Many cross platform applications also require Java to operate properly.

Click on link to go to page where you have s/w to tell you if have Java installed on your PC.
Can also download latest from there.

http://java.com/en/download/index.jsp