AJAX stands for Asynchronous JavaScript with XmlHttpRequest.
How AJAX applications differ from traditional web applications
Traditional web applications allow users to fill out forms, and when these forms are submitted, a request is sent to a web server. The web server acts upon whatever was sent by the form, and then responds back by sending a new web page. A lot of bandwidth is wasted since much of the HTML from the first page is present in the second page. Furthermore, this method cannot create highly interactive user interfaces that work like those of native applications.
AJAX applications, on the other hand, can send requests to the web server to retrieve only the data that is needed, usually using SOAP or some other XML-based web services dialect, and using JavaScript in the client to process the web server response. The result is more responsive applications, since the amount of data interchanged between the web browser and web server is vastly reduced. Web server processing time is also saved, since a lot of this is done on the computer from which the request came.
Adoption of AJAX
Though it has existed for some time, in early 2005 a number of seminal events have popularised the technique. Mainly, Google used asynchronous communication as a basis for prominent interactive applications including Google Groups, Google Maps and Gmail.
Download a .NET AJAX Library
Download a .NET AJAX Library from here.