World of Warcraft: What happens to my old expansion when I install a new one?

So, I just bought the game today. And every single expansion. So i want to know what happens to the game itself, The burning crusade expansion and the Wrath of lich king expansion when I install the Cataclysm expansion. Is there like a dedicated place for each in the map? Can I still play the game itself and the older two expansions if I install cataclysm? Or will it be all about cataclysm if I install it? Thank you