ASP.NET Ajax extension is a part of ASP.NET that makes Ajax functionalities possible and effective. ASP.NET Ajax, formerly known as Atlas, is an extension of ASP.NET 2.0. Ajax extension has developed for both client side and server side. The core part of the Ajax extension client library is the networking layer that manages the complexity of making asynchronous calls over XMLHttpRequest.
The most frequently used ASP.NET Ajax server controls are ScriptManager, UpdatePanel , UpdateProgress and Timer. With the help of ScriptManager Control and UpdatePanel control, ASP.NET Ajax can refresh selected parts of the web page instead of refreshing the whole page with a postback. UpdateProgress provides status information for the progress of the download occurring in the UpdatePanel and the Timer control enables you to perform postbacks at a specified interval.