When you hit it, it takes a "picture" of the screen you are on, i.e. if you are just looking at the desktop and hit it, it takes a "picture" of you desktop. You can then paste the image into your preferred image editing software.
THanks, yes I realise this is what it is supposed to do, but mine doesn't! I've tried combining print screen key with alt, fn, ctrl, windows and so on but no result. I just wondered if this particuluar machine has some quirky key combination that I don't know. Or if its broke....!!