A screenshot image captures all activity on the desktop of an operating system. The captured images can be saved and used for demonstration and troubleshooting purposes. Mac OS X comes with a native application called "Grab," which is designed specifically for capturing high-resolution screenshots.
Double-click the hard drive icon to open it.
Double-click the "Applications" folder to open it.
Double-click the "Utilities" folder and double-click "Grab" to open the application.
Select "Screen" from the "Capture" menu to capture the screenshot of the whole screen.
Click "File" and select "Save" to save the screenshot.
The "Grab" application also allows you to take a picture of a specific window or a part of the screen, and its "Timed Screen" option allows you to start a timer, activate part of the screen and take a picture.