The ASP.NET AJAX Control Toolkit is an open-source project built on top of the Microsoft ASP.NET AJAX framework. ASP.NET Ajax Control Toolkit was first released by Microsoft in early January 2006.
You can download the Ajax Control Toolkit from this link
After you download the Ajax Control Toolkit, extract it in a folder. Next, you need to add the toolkit to the Visual Studio Toolbox. You can create an Ajax Control Toolkit tab to the toolbox and name it as Ajax Control Toolkit. Once done, right-click on the tab you have just created and select "Choose Items" and browse to the path in your system where you have extracted the Ajax Control Toolkit. All of the controls inside the Ajax Control Toolkit will appear in the toolbox in your Visual Studio IDE.
How to use Ajax Control Toolkit
The Ajax Control Toolkit contains a set of Extenders. These extenders can be used to build Ajax enabled controls.
1) First we add the ScriptManager control.
<%--ScriptManager control code--%>
<asp:ScriptManager ID="ScriptManager1" runat="server">
<%-- ScriptManager control manages client script for AJAX enabled ASP.NET pages.This enables partial-page rendering and Web-service calls.You have to used this if you want to use ajax control toolkit--%>
Note: We use the register directives for Ajax control toolkit it associates with the project. Tag Prefix can be change. We write these codes in default.aspx page
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
2) When we drag and drop Ajax toolkit extender then above assembly will automatically add with register directives and its main properties : Assembly, Namespace, TagPrefix
<%--ModalPopupExtender control code--%>
<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server">