CSS, what exactly is it?

[Toy shop cop.]

I recently passed a web design diploma, with a good grade, but we hardly even touched CSS. Perhaps it was old course material. Could somebody please give me a quick outline of what it does. Then I'll set about learning it. No links thanks, I already have what I need. Except the incentive to start reading, which I hope will come when I know what it does. Most thanks to all helpers, and a TU.