Java is an Object Oriented Programming language developed by Sun Microsystems. The first version was released in 1995. Initially it was known as Oak but latter it was renamed to java. Java is platform-independent and it has lots of free and open source libraries that makes building enterprise applications easier and faster with higher degree of stability and robustness.
Server Side
Enterprise applications need stability (including backward compatibility), security and high performance. Java/J2EE/JEE is built around those principles. JVM is very powerful and flexible (now it can also run ruby/python using jRuby/jython or so), no other language can match the level of performance (may be some new languages do but they’re not stable enough for enterprise apps) and the vast number of open source modules/libraries available.
Mobile
Android platform uses Java for app development. So, If you know Java, you can easily pick up Android API (little different from native Java API) very easily.
Desktop
Although, it’s not that popular (like Android or JEE) but you could build cross-platform desktop apps using Java.