Originally Posted by Cleo

To take a picture of the whole screen, press Command-Shift-3.
To take a picture of part of the screen, press Command-Shift-4 and drag to select the area you want.

Command-Shift-4, then Spacebar Take a picture of the selected window

To copy the screen capture to the Clipboard instead of saving it as a file, press Control-Command-Shift-3 to copy the entire screen to the Clipboard, or Control-Command-Shift-4 to copy the selected range to the Clipboard.