MacFixit has a tutorial on using Grab…The Built-in Screen Capture tool for the Macintosh.

Getting to know your Mac: Grab: “In this installation of Getting to know your Mac, MacFixIt editors will take a look at a little utility called Grab. Grab lets you take screenshots of your Mac in a variety of different ways. This can be helpful if you are a blogger doing a tutorial or need to show someone an issue you are having for troubleshooting purposes.”

(View the rest of the article at Getting to know your Mac: Grab)