AJAX's fullname is "asynchronism JavaScript and XML". It's part of the "WEB 2.0". The main technique is XMLHTTPRequest. If you base on IE Explore, you may use ActiveXObject; otherwise use XMLHTTPRequest. The origin of this technique is form Outlook, this software support a object named XMLHTTPRequest, it can transport message between Server and Clients, but only little men know that. Google popularize this techenique widely in many regions, such as google suggest, googe map and Gmail. As I know, Microsoft also use AJAX in MSN Space.
AJAX is not only one techenique, it includes Javascript, DOM, XML, CSS and XHTML and so on. You can get new message without refreshing page, so it reduce the burden of the server and cut the time for user's waiting. Up to now, many comapnies explore AJAX library for it, so AJAXpro and MagicAjax came into our eyes for .NET, this library make our coding simple and easy.
For AJAX is a new technique, it still has some problem to resolve. For example, there is not a environment for Javascript to debug, so if the program has error, it's difficult to find error in short time, it's also a reason that I can accept this new technique fully, although I get deep into this field.I know Microsoft has released a software for AJAX, named Islta, which support javascript very well. I think AJAX will be better tomorrow, just like Flash, has a good future tomorrow.