AJAX (Asynchronous JavaScript and XML), or Ajax, is a group of inter-related web development techniques used for creating interactive web applications. A primary characteristic is the increased responsiveness and interactivity of web pages achieved by exchanging small amounts of data with the server "behind the scenes" so that the entire web page does not have to be reloaded each time the user performs an action. This is intended to increase the web page's interactivity, speed, functionality, and usability.
From : http://en.wikipedia.org/wiki/Ajax_(programming)