To simply put ‘Write JavaScript using JAVA’

And in a little more detail:

Compiles JAVA code into JavaScript code

Takes care of cross browser compatibility by generating a browser specific code

Not client side alone. Has a deployable server side components (which is not compiled into JavaScript)

Client communicates to server via GWT-RPC call (if services are written using GWT),

Supports http calls using JSON and XML format

Supports AJAX calls

With JSNI (Javascript native interface) can mix javascript code with java code

java can call javascript method

javascript can call java method