How can I take a screen shot with Mountain lion

COMMAND-SHIFT-3 snaps the whole screen.

COMMAND-SHIFT-4 snaps what you enclose in the crosshairs.

Snaps are saved on the Desktop.

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Command, Shift and 3 keys > All screen

Command, Shift and 4 keys > Choose

Command, Shift, 4 and Space keys > Window

Hold down...

Shift > Command 3 or Shift > Command 4

There are other options like storing the screenshot in the clipboard instead of saving it as a file, but one that is very useful is to press the space bar when the crosshairs appear after pressing Command-Shift-4, and you can then highlight a window and only take a screenshot of it instead of the entire screen and then cropping it later on. This also has the benefit of preserving window shadows, which can look nice in presentations.