AJAX (Asynchronous JavaScript and XML), or Ajax, is a web development technique used for creating interactive web applications. The intent is to make web pages feel more responsive 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 requests a change. This is intended to increase the web page's interactivity, speed, functionality, and usability.
AJAX is asynchronous in that extra data is requested from the server and loaded in the background without interfering with the display and behaviour of the existing page. JavaScript is the scripting language in which AJAX function calls are usually made.[1] Data is retrieved using the XMLHttpRequest object that is available to scripting languages run in modern browsers. There is, however, no requirement that the asynchronous content be formatted in XML.
AJAX is a cross-platform technique usable on many different operating systems, computer architectures, and web browsers as it is based on open standards such as JavaScript and the DOM. There are free and open source implementations of suitable frameworks and libraries.
SOURCE: wikipedia.org