To take the picture of a part of the screen
Command (⌘)-Shift-4, and then drag the crosshair pointer to select the area. Continue to press the mouse button, release the keys, and then press Shift, Option, or the Space bar while you drag to resize the selection area. When you are ready to take a picture, release the mouse button.
Pictures of the screen (screenshots) are saved as files on the desktop, but if you prefer to put a screenshot in the Clipboard, hold down the Control key while you press the other keys. You can then paste the picture into a document.