AJAX (Asynchronous JavaScript and XML) is a set of programming techniques that allow Web applications to be much more responsive and provide usability on par with traditional client/server applications. Using AJAX techniques, applications can act on data and events as the user interacts with the system by communicating to the server and updating portions of a page immediately instead of always refreshing the entire page. The results are faster, more robust applications.

Allows developers to quickly build interactive, event-driven functionality utilizing the power of Java on the server and JavaScript on the client.

ajax is nothing but asynchronous javascript and xml

following r the features of ajax:

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every time page posted back to server the page refreshment takes.for instance when user clicks a button on page all the controls on page will be posted back to server which is not required.with the ajax one can send part of the page to server i.e partial refreshment of page.