Ajax stands for "Asynchronous JavaScript and XML". Once merely a party trick of Web 1.0 sites, Ajax has now firmly established itself as the wunderkind component of interactive Web 2.0 and Web 3.0 sites. Despite its impressive resumé, don't forget that on a functional level, everything that is done with Ajax can also be done without Ajax—its only function is to enhance user experience. Any site built with Ajax can be fully operational without Ajax, albeit with more mouse clicks.
The following is a simple but relevant example: go to http://www.pandora.com or http://www.google.com and try searching for something. You'll notice that search results appear without you having to submit the form.
You didn't have to click anything. This is the magic of asynchronous JavaScript. You received your search results and the page did not have to reload. We have all been on websites where we had to click a button to submit a form, but the user experience is smoother when Ajax is used.
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