From http://en.wikipedia.org/wiki/Google_Web_Toolkit:
The Google Web Toolkit is a free toolkit by Google to develop Ajax applications in the Java programming language. GWT supports rapid client/server development and debugging in any Java IDE. In a subsequent deployment step, the GWT compiler translates a working Java application into equivalent JavaScript that programatically manipulates a web brower's HTML DOM using DHTML techniques. GWT emphasizes reusable, efficient solutions to recurring Ajax challenges, namely asynchronous remote procedure calls, history management, bookmarking, and cross-browser portability.