The Java Runtime Environment software by Oracle 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. The program's license is completely free.
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.
In case you're looking for JDK or Java Development Kit, you can download it here.