Command-Shift-4 – Will bring up a target icon. Select the area you want to take a screenshot of and it will save that selection to your desktop.
Command-Shift-3 – Will take a screenshot of what is on your current screen.
Command-Shift-4, then space, then click a window – Will take a screenshot of the selected window.
The following commands will do the same thing as above but will save your picture to the clipboard for use wherever you want to paste it to:
Command-Control-Shift-3 – Will take a screenshot what is on the screen and save it to your clipboard
Command-Control-Shift-4, then select an area – Will take a screenshot of the selected area and be saved to the clipboard.
Command-Control-Shift-4, then space, then click a window – Will take a screenshot of the window you select and save it to the clipboard.