Ajax is a new way of making the websites respond faster. I have developed applications in the past and one of my constant worries was how to make the site respond faster. When I used to look at the response time of gmail , orkut and google suggest, I did wonder what makes them so fast. Now I know the answer :Ajax

Here are two links which explains the technology in simple terms:
adaptivepath.com/publications/essays/archives/000385.php
serversideguy.blogspot.com/2004/12/google-suggest-dissected.html