Mac OS X Screenshot Tutorial
From Wikipedia, "Pressing Command-Shift-3 takes a screenshot of the entire screen, and Command-Shift-4 takes a screenshot of a chosen area of the screen or if you press Space afterwards you can choose a window on the screen to individually screenshot. These images are saved to the desktop, but if you hold down the control key with the rest of the keyboard shortcut, the pictures are copied to the clipboard instead. These shortcuts also work in Mac OS Classic.
You can also use the Grab application to take screenshots.
A shell utility called "screencapture" (located in /usr/sbin/screencapture) can be used from the Terminal application or in shell scripts to capture screenshots and save them to files. Various options are available to choose the file format of the screenshot, how the screenshot is captured, if sounds are played, etc. The manual page (available via the command "man screencapture") explains all the options. This utility might only be available when the Mac OS X developer tools are installed."