How do I take a screenshot on my Mac?
cmd  shift  3 takes a screenshot of the entire current Desktop (all attached monitors, as separate shots].

cmd  shift  4 takes a screenshot of a dragged selection.

cmd  shift  4 then space (you must press space separately) then click, to take a screenshot of a specific window.
Holding  Opt ⌥   as you click will remove the drop-shadow.

Add control to the shortcuts above to place the screen shot on the clipboard instead of saving it to the desktop.