Print Screen Key alone = captures entire screen to clipboard
CTRL and Print Screen Key together = captures entire screen to clipboard
ALT and Print Screen Key together = ONLY captures Active window to clipboard

Shift AND Print Screen Key together = printing screen shot directly to the printer BUT only in PRE WinXP versions of Windows.

Once a screen shot is on the clipboard, open WORD, Paint or any program of your choice that will accept input graphics and press CTRL-V (paste) and your screen shot will paste in the program.

By the way, if you press ALT SHIFT and PRINT SCREEN together at the same time, you will change your screen resolution to High Contrast (if you click OK)

Refresher keyboard shortcuts:
Press the CRTL key and the letter C to copy
Press the CRTL key and the letter V to paste
Press the CRTL key and the letter X to cut
Press the CRTL key and the letter B for BOLD
Press the CRTL key and the letter I for Italics
Press the CRTL key and the letter U for Underlined
Press the CRTL key and the letter Z to undo a change
etc. etc.