1. Full Screen
To capture the full screen, hold down command then shift and press 3.
2. Capture a Part of Your Screen
Hold down command, then shift and 4. Crosshairs will show up and you can draw out the portion of the screen you want to screenshot.
3. Copy and Paste Using the Clipboard
Hold down the Control key and either use “Shift+3″ or “Shift+4″ to get the screen copied to your clipboard.
Once copied to the clipboard, press Command and V to paste the screen.