No, it is just Javascript, but it is sophisticated javascript that supports the integration of the client and the server without a page refresh.
I'm currently building ajax into an intranet site, I have smidgens on other sites and can see it being used more and more. Before you start though it pays to understand the usability and SEO consequences on your design.
There are a great many AJAX libraries but the two I've looked most closely at are
script.aculo.us
xajax
I'm using script.aculo.us because my needs are relatively simple and it's easy to integrate with
CakePHP. However I suspect the actual structure for building an AJAX site are better with xajax.