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shift + command + 3 = Whole screen shot
shift + command + 4 = selection screen shot
shift + command + spacebar + 4 = window screen shot


Or if you need timed screenshots then you can use Grab.app in your utilities folder.
 
ctrl+cmd+shift+3 copies full screen, and you can paste it in as an image in photoshop, keynote, powerpoint etc.

ctrl+cmd+shift+4 copies selection
 
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