Configuring ASP.NET AJAX in the web.config

Introduction

This topic describes the elements in the Web.config file that support Microsoft ASP.NET AJAX. It also describes how to incorporate those elements into the Web.config file for an existing ASP.NET application.

Please access the following URL to get more detail.
http://asp.net/AJAX/Documentation/Live/ConfiguringASPNETAJAX.aspx