http://osxdaily.com/2010/05/13/print-screen-mac/

  • Command+Shift+3 takes a screenshot of the full screen (or screens if multiple monitors), and save it as a file to the desktop
  • Command+Shift+4 brings up a selection box so you can specify an area to take a screenshot of, then save it as a file to the desktop
  • Command+Shift+4, then spacebar, then click a window: takes a screenshot of a window only and saves it as a file to the desktop