AJAX is (yet another) acronym which is short for “Asynchronous JavaScript + XML”.
But what AJAX is about is using a group of technologies which are already out there and which are well understood, and combining them in order to provide better, more responsive and more powerful web applications.
In a nutshell AJAX makes use of the following technologies:
XHTML and CSS for presentation and design;
The DOM for interaction with and dynamic display of data;
XML (and, of course, XSLT for data manipulation and transmission;
XMLHttpRequest for asynchronous data fetching;
JavaScript to bind it all together and control everything.
Resources
Ajax: A New Approach to Web Applications - An excellent introduction to the whole concept of AJAX. This is where I got most of the information from for my first foray into it.