what is Ajax

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