Comes with OS X
Taking pictures of your screen with Grab
You can use the Grab application to take a picture of all or part of your computer screen. These pictures are often called "screen shots" or "screen captures."
Open Grab (located in /Applications/Utilities) and choose a command from the Capture menu.
Selection takes a picture of a part of the screen that you select.
Window takes a picture of a window you select.
Screen takes a picture of the entire screen.
Timed Screen lets you start a timer and activate part of the screen (such as a menu), and then takes a picture of the screen.
Follow the instructions in the dialog for the command you chose.
To see information about a screen shot you've taken, such as the size and depth, choose Edit > Inspector.
To save a screen shot, choose File > Save. Grab saves screen shots as files in TIFF format. You can use the Preview application to view these files and export them to other formats, such as JPEG.