HT1343: OS X keyboard shortcutsLearn about OS X keyboard shortcuts
Currently Being ModeratedDec 12, 2012 4:09 PM (in response to bobfromhull)
⌘ shift 3: takes a screen shot and saves it to the desktop.
⌘ shift 4: allows you to select the target area of the screen you wish to capture.
⌘ shift 4 spacebar: allows you to capture a window.
Here's the Knowledge Base article.
The Grab application in the Utilities folder also allows you to perform screen captures.