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), established 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.