I'm using Windows 8.1, and WIN + PRTSC takes a shot of the entire screen and stores it in my Pictures folder, in Screenshots. ALT + PRTSC, on the other hand, copies an image of the currently active window only to the clipboard, whence I can paste it into Paint (or a word processor, or whatever) and save it from there. I don't know about previous versions of Windows, but 8 also has what's called the Snipping Tool, which allows you to select whatever you want from the current screen and save it. Of course, if you're using Win 7 or earlier, any or all of the above might be different for you.

There are also third-party programs -- some free -- which can take screenshots and allow you to do specific things with them, but, until quite recently, I've never had any need or desire to take screenshots, so I can't recommend anything. (And if all you're trying to do is take simple screenshots of your Terraria creations, an extra program just for that might be overkill anyway.)

If all else fails, remember Google is your friend! =)