Created by Sun Microsystem, Java is one of the most used modern programming languages.
You see it everywhere possibly without even knowing it - applets or games on a webpage or on a cell phone, thousands and thousands of websites whose backend is written in Java, etc.
One of the key advantages that Java has over other languages is that Java is pretty much platform-independent. It certainly or nearly lives up to the 'Write once, run everywhere' jargon.
Another advantage is that Java is supported by so many useful packages that perform their specialized functions such as JCE (Java Cryptography Extension), RMI (Remote Method Invocation), Servlet, image processing library, etc.
You can take a look at the latest Java API, and you will see a huge list of classes you can use.
In school I used Java to complete many of school assignments. My first several jobs also revolved around Java. We used Java to build enterprise websites. The first one is GoToMeeting and GoToWebinar. The second one is Like.com which was acquired by Google in 2010.
The popular Java framework, the Spring Framework, made Java even more popular because it makes software developers focus only on the business logic and not have to worry about boilerplate code.
One of the commercial projects I've done is use Java to develop a GUI that interfaces with nCipher's HSM, or Hardware Security Module. Read on!