In addition to the Grab application, you can also use keyboard shortcuts to take pictures of the screen in Mac OS X.
To take a picture of the whole screen, press Command-Shift-3.
To take a picture of part of the screen, press Command-Shift-4 and drag to select the area you want.
The picture is saved as a TIFF file on your Desktop. Note: Mac OS X 10.2 and later saves pictures as PDF files. See document 107078 “Mac OS X 10.2: Screen Shots Are Saved in PDF, May Be Converted” for additional information.
To copy the screen capture to the Clipboard instead of saving it as a file, press Control-Command-Shift-3 to copy the entire screen to the Clipboard, or Control-Command-Shift-4 to copy the selected range to the Clipboard.