Hold down the Command, Shift, and 4 keys, then press the Spacebar. Instead of a cross-hair cursor, a small camera icon appears. When you move this camera icon over the element you’d like to capture, that element is highlighted. Click your mouse or trackpad, and you’ve captured a screenshot of just that element — no further cleanup required.
The image will be stored on your desktop, named Picture1, Picture2 and so on.
This trick is nice.