Auto Resizing?

How would I get a page to do what this one does when you drag the size of the window around (resizes the center content frame to adjust rather than outside):

http://www.bjork.com/

?

I know how to get these things done on the outside (keeping things centered), but I can't seem to make the leap between that and this.

If you don't feel like writing all out how it's supposed to be done, a link is fine, or if someone could at least tell me what that effect might be referred to as, it would be greatly appreciated.

Thanks!