AJAX is not a technology in itself. It is a Web development technique for creating Web application by combining technologies.
The AJAX technique for Web application development enables you to overcome the page loading requirements of traditional HTML/HTTP Web pages. Because Web pages are not tightly bound to data sources and because the data needs to be formatted before it can be displayed on a Web page, the Web page needs to be reloaded every time a user wants to view different or refined data. The XmlHttpRequest object allows a Web page to request and return data without having to reload the page.