How Did They Do That?

Can anyone point me in the right direction on how to do a transitional effect like the one on:

http://www.marines.com

I am trying to do my menu like the one at the top, where the tabs are titled, “Those Who Are Warriors, Those Who Are Driven,” etc. When you press the tabs they all do a wiping effect of the one before that. Are those all done as separate movie clips? Any help would be great!