Well its a relatively new coding language, based around Java (I think) or html...

It can automate things without reloading the page.

For example, vB have used it in thier quick reply boxes. You type your reply in the quick reply box, click post, a message will appear below saying 'Please wait - Posting Quick reply', then within 2 seconds the page updates (without a refresh) automatically with your reply.

There are quite a few other example of this, but AJAX is capable of so much.