In Mac OS X, you can use simple keyboard shortcuts to do all sorts of things, including capturing images of what’s on your Mac screen. For example, you can take a screen shot of your entire screen by holding down the Command and Shift keys and pressing a 3.
If you hold down the Command and Shift keys and press 4, Mac OS X turns the cursor into crosshairs you can use to select whatever portion of your display you’d like to capture in a screen shot.
If you immediately hit the Spacebar after typing Command-Shift-4, Mac OS X replaces those crosshairs with a little camera. Using the camera, you can take a screen shot of the Dock, the entire menu bar, a single open menu, the desktop or any open window.