AJAX is a new buzz word given to an old idea.  AJAX stands for Asynchronous JavaScript and XML.  The idea is to use webservices and the XMLHttpRequest  object to provide a more interactive web experience.  By combining these two technologies, we can add server side changes to a web page with out having to refresh the web page, making it appear more interactive with the user.

All of the major browsers support  methods for making client side HTTP requests.  This includes Mozilla/FireFox, Internet Explorer, Safari, Opera, Konqueror, and IceBrowser.

One of the best uses and examples that I have seen of AJAX is Google Maps.  You can really see how effective AJAX can be when used properly.

AJAX promises to be a large part of Microsofts new development environment, and will quickly catch on through out the web.
See more information on AJAX in my AJAX Section