Given that Sun's slogan "the network is the computer" has guided
Java development, you have a huge field to study.
Better pick one of the simpler ones for a start. I am fond of Java Message Service JMS as a simple example.
Here is Sun's current start page for JMS
Here is the Apache open source version.
Both of those have accumulated lots of complex examples over the years, try to start with simple Publish/Subscribe message distribution.
Bill