For applying AJAX functionality to the ASP.Net web application you have to install the ASP.Net AJAX Extension libraries that will add a new template providing support for AJAX and preconfigured web.config file AJAX enabled web site. After installing ASP.Net AJAX extensions you will get new template into create new web site options i.e. AJAX enabled ASP.Net web site. Another thing Toolbox will show new Tab AJAX loaded with AJAX extension library components as following:

1. Timer
   
   
2. ScriptManager
   
   
3. ScriptManagerProxy
   
   
4. UpdatePanel
   
   
5. UpdateProgress

These are the basic components of AJAX library that integrate with ASP.Net server code to provide rich user experience and most user friendly UIs.

Following are the most frequently used controls of AJAX extensions:

ScriptManager

ScriptManager provides all the client script libraries to enable the ASP.Net web site work with AJAX functionality and components. ScriptManager is added at the top of ASP.Net web page so that it could include all the resources required for client components, partial-page rendering, localization, globalization, web services and custom client scripts before loading any AJAX control.

UpdatePanel

UpdatePanel enables you to perform partial page postbacks by refreshing only the part of web page without reloading the whole page that increases the applicartion efficiency and performance by reducing the network load.

UpdateProgress

UpdateProgress provides the feature to display the status information about partial-page updates. You can display the text messages or animated loading symbols to show the status of back-end processing.

Timer

Timer control has the functionality to set the time intervals for page postbacks. You can use Timer control together with UpdatePanel to refresh the partial page.

You can create your own AJAX extender controls according to your requirement in ASP.Net applications. Official Website of Microsoft http://www.asp.net/ is also providing AJAX Control toolkit that contains most of the AJAX based extenders.