AJAX [Asynchronous JavaScript & XML] is not a new programming language, but a new way to use existing standards. AJAX is the art of exchanging data with a server, and update parts of a web page - without reloading the whole page. AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.
In today’s competitive internet world, web applications need to develop high-speed and interactive development where Ajax programming make easy to conquer the page loading requirements of HTML/HTTP mediated web pages.

AJAX is based on internet standards, and uses a combination of:

XMLHttpRequest object (to exchange data asynchronously with a server)
JavaScript/DOM (to display/interact with the information)
CSS (to style the data)
XML (often used as the format for transferring data)

AJAX is taken as general part of any dynamic web page with advanced user experience, which has benefited from the more sophisticated functionality using easier-to-implement web standards. Due to its added functionality, this technology is proving to be a real alternative for creating powerful web applications.
Through the benefits of these technology, let develop lot of web applications like Content management system, image gallery application, shopping card etc.