Wow. That's a big question.
In short, Java is an object-oriented programming language developed by Sun Microsystems. It is freely available and provides many APIs for networking programming, XML processing, XML, security, string manipulation, etc.
Java provides a rich features set for client side (applets, custom clients), Web (servlets, Java Server Pages) and application server tier(sockets, remote method invocation, transaction processing, Java database connectivity)programming.
The great thing about Java is that the compiled byte-code is cross-platform and will run on Solaris, Windows, Linux, MacOS, among others and to a lessor degree Palm and CE. If you are going to be deploying an app on multiple platforms, Java is a wonderful language to use.
For more information go to http://java.sun.com/nav/whatis/ to get a complete overview on the platform.
This was first published in August 2001