AJAX is based on JavaScript and HTTP requests. It is a type of programming made popular in 2005 by Google (with Google Suggest). It is not a new programming language, but a new way to use existing standards. It is a group of interrelated web development techniques used to create interactive web applications or rich Internet applications. With Ajax, web applications can retrieve data from the server asynchronously in the background without interfering with the display and behavior of the existing page. Data is retrieved using the XMLHttpRequest object.

Ajax uses a combination of:

• HTML and CSS for marking up and styling information.

• The DOM accessed with JavaScript to dynamically display and interact with the information presented.

• A method for exchanging data asynchronously between browser and server, therewith avoiding page reloads. The XMLHttpRequest (XHR) object is usually used, but sometimes an IFrame object or a dynamically added <script> tag is used instead.

• A format for the data sent to the browser. Common formats include XML, pre-formatted HTML, plain text, and JSON. This data could be created dynamically by some form of server-side scripting.

One example of AJAX riched website is FACEBOOK.

source: http://en.wikipedia.org/