You should not use AJAX if:

             =>  You want the page to show in a search engine, such as Google, because WebCrawler does not execute JavaScript code.

             =>  The browser does not support JavaScript.

             =>  You want to create a secure application.

The following validation controls are available in ASP.NET AJAX 4.0:

        => FilteredTextBoxExtender - Enables you to apply filtering to a text box.

        => MaskedEditExtender and MaskedEditValidator - Restricts a user to enter only a certain pattern of characters in the TextBox by applying a mask to the input.

        => ValidatorCalloutExtender - Attaches to the ASP.NET validators so that the error messages are not displayed as a simple text but as a balloon-style ToolTip.

        => NoBot - Prevents the spam/bot from filling the input forms automatically and uses the Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHA), which is a type of challenge-response test to ensure that the response is not generated by the computer.

        => PasswordStrengthExtender - Measures the strength of the password text entered within the text box by validating with the different strength specified parameters

AJAX uses four technologies, which are as follows:

       => JavaScript

       => XMLHttpRequest

       => Document Object Model (DOM)

       => Extensible HTML (XHTML) and Cascading Style Sheets (CSS)

ASP.NET AJAX, mostly called AJAX, is a set of extensions of ASP.NET. It is developed by Microsoft to implement AJAX functionalities in Web applications. ASP.NET AJAX provides a set of components that enable the developers to develop applications that can update only a specified portion of data without refreshing the entire page. The ASP.NET AJAX works with the AJAX Library that uses object-oriented programming (OOP) to develop rich Web applications that communicate with the server using asynchronous postback.