Introduction:
ASP.NET is a set of web development technologies produced by Microsoft that is used to build dynamic web sites, web applications, and XML-based web applications. ASP.NET is a part of the .NET framework and allows for developers to build applications in multiple languages, such as Visual Basic .NET, JScript .NET, and C#.

Main:

Advantages of AJAX

The advantages of AJAX over classical web-based applications include:

Asynchronous calls— AJAX allows for the ability to make asynchronous calls to a web server. This allows the client browser to avoid waiting for all data to arrive before allowing the user to act once more.

Minimal data transfer— By not performing a full postback and sending all form data to the server, network utilization is minimized and quicker operations occur. In sites and locations with restricted pipes for data transfer, this can greatly improve network performance.

Limited processing on the server— Along with the fact that only the necessary data is sent to the server, the server is not required to process all form elements. By sending only the necessary data, there is limited processing on the server. There is no need to process all form elements, process the ViewState, send images back to the client, or send a full page back to the client.

Responsiveness— Because AJAX applications are asynchronous on the client, they are perceived to be very responsive.

Context— With a full postback, users may lose the context of where they are. Users may be at the bottom of a page, hit the Submit button, and be redirected back to the top of the page. With AJAX there is no full postback. Clicking the Submit button in an application that uses AJAX will allow users to maintain their location. The user state is maintained, and the users are no longer required to scroll down to the location they were at before clicking Submit.

Advantages of Asp.Net 2.0 Ajax:

AJAX-style communications between client and server. This communication is over web services.

Asynchronous communication. All client-to-server communication in the ASP.NET 2.0 AJAX framework is asynchronous.

A set of server-side controls that enable rich client-side functionality.

A set of client-side controls and classes that further enable client-side functionality.

A framework for encapsulating client-logic through the creation of namespaces and classes.

Cross browser support.

Installing ASP.NET AJAX:

Before you can start using ASP.NET AJAX, you need to install it. You can find the latest installation at the official Microsoft ASP.NET AJAX web site, http://ajax.asp.net/. From there, click the Download link. You may also wish to download the latest ASP.NET AJAX Control Toolkit and ASP.NET 2.0 AJAX Futures CTP, because these provide additional features beyond the ASP.NET AJAX core installation.

After you have downloaded the installation files for ASP.NET AJAX, run the installer (ASPAJAXExtSetup.msi).

After accepting the End User License Agreement, click the Install button,

Wait for the installer to complete, which should take just a few minutes. When it has completed, you should see a screen confirming that the installation was successful. You may wish to read the Release Notes at this time; if so, just leave the Display Microsoft ASP.NET 2.0 AJAX Extensions 1.0 Release Notes check box checked. Click the Finish button to complete the setup process.

The installation process installs ASP.NET AJAX in the Global Assembly Cache (GAC) of the computer. In most environments, the ASP.NET AJAX assembly (System.Web.Extensions.dll) should not be included in the /bin folder of your web site, but rather ASP.NET AJAX should be installed for the entire server.

Conclusion:
Hope this helps,
Happy coding.