Drop Down Menus- Horizontal

Does anyone know how to, or of any tutorials that have drop down menus like this:

http://www.kalou.ch/fr/

I don’t mean the way it moves around, but how the sub-menus load underneath.

I want to create a horizontal navigation where you click on a main button, and a sub-menu loads underneath the main navigation. Then I want the same for the other buttons. But I can’t figure out how to get the first sub-menu to unload when the new sub-menu loads.

Does this make sense?

Thanks for any help in advance,

Chris.