..Will it actually print the screenshot?
No, it will make a copy of whatever is on your screen at the moment. (makes an image, not actual print)
So if you press it while playing sims, it will take a picture of everything visible on the game screen.
You will have to 'paste' and save it as an imagefile tho to use it.
(sorry if this is not a very good explanation - english is my second language and I don't know too much tech related vocabulary)