Its really, fascinating interactiveness provides to the users on the client-side browsers, without submitting the pages on request and allow them to wait till the browser refresh the window with the response.
The AJAX is implemented succesfully in the websites lime Google-Mail, Google-Maps, Flickr, Amazon’s A9.com search engine & many more.
For more information read these links:
ajax: a new approach to web applications
AJAX - Wikipedia, the free encyclopedia