http://www.adaptivepath.com/publications/essays/archives/000385.php

Quoted from above web site.

Take a look at Google Suggest. Watch the way the suggested terms update as you type, almost instantly. Now look at Google Maps. Zoom in. Use your cursor to grab the map and scroll around a bit. Again, everything happens almost instantly, with no waiting for pages to reload.

Google Suggest and Google Maps are two examples of a new approach to web applications that we at Adaptive Path have been calling Ajax. The name is shorthand for Asynchronous JavaScript + XML, and it represents a fundamental shift in what’s possible on the Web.

Another website with an Ajax example at

http://java.sun.com/developer/technicalArticles/J2EE/AJAX/

Website for CSS and Javascript:

http://www.quirksmode.org/