Grab is a tool built in to OS X that is very handy and many will find expendable. It basically lets
you capture an area of your screen as an image file. Try it our using the below shortcut.
Take a screenshot of the screen, and save it as a file on the desktop
Command (Apple) & Shift & 3
Take a screenshot of an area and save it as a file on the desktop
Command (Apple) & Shift & 4, then select an area
Take a screenshot of a window and save it as a file on the desktop
Command & Shift & 4, then space, then click a window
The Grab application can be found in the following location. Applications>Utilities>Grab