I am planning to develop a website, i want it to be in Ajax to make it more fast. can some one suggest me what is actually GWT ? is it a front end technology only ? can i integrate this with any other server side projects or not?
GWT is a client side technology. You write Java code and the GWT compiler will convert it into JS. It will also optimize it for different browser/versions.
GWT provides a RPC mechanism which you can use to make asynchronous server side calls. You can integrate it with your existing project on the server side, provided you use your server side code for data manipulation and not page manipulation.
Recommended reading http://code.google.com/webtoolkit/overview.html