This question is near-equivalent to "tell me what is AJAX", because the whole purpose of AJAX is to improve performance of a Web page, where "performance" is understood in a very wide sense of this word.
The benefits and drawbacks of AJAX-based technology are explained here:
http://en.wikipedia.org/wiki/Ajax_%28programming%29[
^].
AJAX is the only client-side method totally based on pure HTTP which allows the client-side to send some HTTP request abstracted from the whole Web page or any other resource. In other words, you can send a request to the server side asking for the value for inner text of only one inline element. For example, you can ask for a value of only one text label, depending on some parameters you send in an HTTP request. To understand how it looks, please see the code sample in the article referenced above.
As a result, the page does not "flicker" due to replacement of the content of a whole page. Instead, JavaScript receives a new value in a callback (asynchronous processing is the important AJAX feature, so the page remains responsive during the whole request/response trip) and assigns new value to the DOM object.
—SA
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