I want to know that can we use GWT with java web applications or it is totally different technology?
GWT focuses on the client-side, and all code on the client-side is compiled to JS. The only interaction with server-side resources would be through RPC calls. With GWT you get a few tools to help with that...
* RequestBuilder - REST style call, just like you would with Prototype, or other AJAX framework.
* XML-RPC - Specific to GWT this allows you to pass Java objects between the client and a server-side servlet. It automatically handles the object serialization.
* XML Parser - The XML parser that comes with GWT can be used to parse incoming requests fetched with RequestBuilder. There is a third-party library that used this to create a XML-RPC library.
Hope that helps.