GWT allows the creation of Ajax web applications using only Java code. In hosted development mode your GWT appliction code continues to run as Java inside the JVM. This allows developers to leverage their existing IDE's for functionality like profiling and debugging. At deployment time GWT project code is compiled into Javascript form and ready to run in a browser. UI development features a Swing like widget API which is event driven. GWT also adds an RPC layer that shields developers form the complexities of using the XMLHttpRequest object.
GWT has been gaining support in the Java community since its release. Sites are already appearing featuring integration with third-party Javascript API's such as Script.aculo.us. Support of GWT has also been announced for IntelliJ and for Netbeans.