We all experienced the power of AJAX. May it be any web platform you are using for developing a web application, you can work with AJAX. With regards to ASP.Net we have AJAX Control Library which provides us with extensions and also with controls which makes using AJAX in our web applications a breeze.
Basically, ASP.Net is a technology which is server based and all the processing happens at server end. At times you need things to work at client end but still wish to utilize the power of server side processing which is inherited by the ASP.Net server controls. These controls are the ASP.NET AJAX representations of DOM elements, allowing you to program against these elements using the ASP.NET AJAX Framework. So now you harness the power of server controls for your client controls.
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Basically, ASP.Net is a technology which is server based and all the processing happens at server end. At times you need things to work at client end but still wish to utilize the power of server side processing which is inherited by the ASP.Net server controls. These controls are the ASP.NET AJAX representations of DOM elements, allowing you to program against these elements using the ASP.NET AJAX Framework. So now you harness the power of server controls for your client controls.
More on this read this