Ajax is a collection of technologies that allow uses to interact with a web site, and the web site to interact with a server, without post-backs and server round trips. Thus providing a richer user experience. Before I go on, try it out and see what you think.
Google maps : http://maps.google.co.uk - great dynamic maps you can drag about, with neat integration to the search engine. As you drag to other parts of the map, Ajax gets the new parts of the map for you from the server.
A9 search : http://a9.com - Amazons search engine, try the column buttons on the left hand...