AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes.
The keystone of AJAX is the XMLHttpRequest object.
All modern browsers(IE7+, Firefox, Chrome, Safari, and Opera) support the XMLHttpRequest object (IE5 and IE6 uses an ActiveXObject).
The XMLHttpRequest object is used to exchange data with a server behind the scenes.
Applications without AJAX
Normal Web applications communicate with the server by referring to a new URL
Example: When a form is submitted, it is processed by a server-side program, which gets invoked
HTML form is not needed to communicate with the server.