[manasecret]

EDIT: Oops, Pylon beat me to it. Ah well.

Turn-based strategy, such as Civilization. In Civ, you get to move your entire turn while the computer waits, and then the computer gets to move its entire turn while you wait, and so on.

Real-time strategy (ala Age of Empires) is one in which every player is playing all at once, things happening in real time. When someone attacks you, you don't wait until your turn to react -- you react as soon as he attacks. And so on.

I guess you probably know all that, but it's fun trying to explain it.