I’ve had reason to capture rather a lot of screen shots in recent weeks and find the following Mac keyboard shortcuts very useful:
|Command-Shift-3||Captures entire desktop (saved as PNG file on desktop)|
|Command-Ctrl-Shift-3||Copies entire desktop (saved to your paste buffer)|
|Command-Shift-4||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.