I’ve had reason to capture rather a lot of screen shots in recent weeks and find the following Mac keyboard shortcuts very useful:
|Captures entire desktop (saved as PNG file on desktop)
|Copies entire desktop (saved to your paste buffer)
|Captures portion of desktop (saved as PNG file on desktop)
|Command-Shift-4 + Spacebar
|Captures current window (saved as PNG file on desktop
There are lots of other variations on the above, which are described nicely here.