How do they do that?

Does anyone know how they do the sliding, horizontal, transitions on the following web page:

http://www.xsilva.com/launch_2.5.php

I mean the part at the bottom, below the mac and ‘X’, when you click on one of the 7 tabs.

It would be much appreciated if you could point me to a tutorial or explain the basics (then again it ay not be that basic!).