Java is one of the most modern programming languages around with a large community of developers and supporters. Invented and driven by Sun Mircosystems Inc.Java became very popular and a lot of features were developed around it.
Java needs a so called "Java Runtime Environment" to run the programs. This environment is available for almost all operating systems and therefore, there is not limitation to use it. Java was also futher enhanced for embedded and mobile devices as JavaME (Java Micro Edition) and for large enterprise application server environments as JavaEE (Java Enterprise Edition).
Java is a very good choice for Enterprise Environment with Application Servers like JBoss. Java can also be used to write GUIs (Graphical User Interfaces) for client server applications or as front ends for command line tools.
For more information on Java have a look to the official website: http://java.sun.com.