Mac OS X gives you the ability to grab a screenshot of your monitor using the following combination:
Also it provides the capability to grab only a portion of your screen using the following combination:
After you type this combination simply click and grab the mouse pointer to select the desired portion of screen.
This two tips are very simple but they generate images that often you need to modify to remove some portions you don't need.
If you need to grab only specific elements of your GUI you can use the following combo: