The printscreen button takes a bmp snapshot and stores it in the clipboard.
You can paste it in a graphics program like mspaint and save it in some more space-saving format. GIF is perfect for screenshots of user interfaces.
Note that with Alt+PrintScr you can also make a snapshot of only the foreground window, which can be usefull for making screenshots of property windows.
For the old-style Printscreen in DOS programs that streamed ASCII to LPT ports I don't know a solution unfortunately.
Hope this helps