AJAX (Asynchronous JavaScript and XML), or Ajax, is a group of inter-related Web development techniques used for making interactive Web applications. A main characteristic is the increased responsiveness and interactiveness 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.
AJAX is asynchronous in that extra data is requested from the server and loaded in the background without interfering with the display and behavior of the existing page. JavaScript is the scripting language in which AJAX function calls are usually made. 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.
The core justification for Ajax style programming is to overcome the page loading requirements of HTML/HTTP-mediated Web pages. Ajax creates the necessary initial conditions for the evolution of complex, intuitive, dynamic, data-centric user interfaces in Web pages—the realization of that goal is still a work in progress.
Web pages, unlike native applications, are loosely coupled, meaning that the data they display are not tightly bound to data sources and must be first marshaled (set out in proper order) into an HTML page format before they can be presented to a user agent on the client machine.
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