JSON is an abbreviation of JavaScript Object Notation. It is a safe and reliable data interchange format in JavaScript, which is easy to understand not only for the users but also for the machines.

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

The following are the limitations of AJAX:
• It is difficult to bookmark a particular state of the application.
• Function provided in the code-behind file do not work because the dynamic pages cannot register themselves on browsers history engine automatically.
• If JavaScript is disabled, then AJAX is not able to perform any work.
• Response time may be slow because different controls of a page are loaded at different time.

The differences between AJAX and JavaScript are given as follows:
• AJAX sends request to the server and does not wait for the response. It performs other operations on the page during that time. JavaScript make a request to the server and waits for response.
• AJAX does not require the page to refresh for downloading the whole page while JavaScript manages and controls a Web page after being downloaded.
• AJAX minimizes the overload on the server since the script needs to request once while JavaScript posts a request that updates the script every time.

The UpdatePanel control specifies the portions of a Web page that can be updated together. As theUpdatePanel control refreshes only a selected part of the Web page instead of refreshing the entire page with a postback, you get more flexibility to create rich and client-centric Web applications.

Refreshing a selected part of the Web page is referred as partial-page update. You can add one or moreUpdatePanel control in the Web page, which automatically participates in partial-page update without custom client script. The UpdatePanel control uses the UpdatePanel class to support the partial-page rendering.