Ajax is a new technology (in fact a combination of 'old' technology's) to drive your website. (I always thought it was our famous dutch football team from Amsterdam)
Ajax is hot(or hype). But what is it?
For a good introduction and (.NET) example you better check out this dotnetbips article.
More on the technology can be found in this article.
Marco Cantu gave a BOF session on Ajax and Delphi at the BorCon, the slides can be found here.
Certainly will give this a try one day.