Ajax is not a technology or a product , it has introduced a new way of using existing standards. Ajax is not specific to ASP.NET, Java or PHP, you can use Ajax in these preceding technologies. Ajax provide cross-platform, cross-architecture, and even cross-browser support.
Now a days Ajax is very popular among web development because Ajax applications can update WebPages without doing a full postback to the web server, it is called Partial-page rendering. Partial-page rendering that refresh only the parts of the Web page that have changed and avoid complete page refresh. Partial-page updates allows the user to avoid waiting for all data to arrive before allowing the user to do the next action. From the following chapters you can learn more about Ajax in detail.