How do they do this?

[Aries Mars]

istockphoto: - look at their popups on mouseover.

I guess its done with AJAX but... well I guess I would like to know where to start to get a version going of my own, anyone know of any good tuts? I Googled and Yahooed for the awnser, but to come to think of it, I dont even know what it's called. Pop-up windows? Hmm, I'll keep looking but if anyone can give me some pointers it would be great. Thx.

[IAIHMB]

This has actually been brought up once before. An example was provided, a bit to obtrusive for my liking but different strokes.