How to take a screenshot on a mac

OS X Mountain Lion: Shortcuts for taking pictures of the screen

Pictures of the screen (screenshots) are saved as files on the desktop, but if you prefer to put a screenshot in the Clipboard, hold down the Control key while you press the other keys. You can then paste the picture into a document.

Action	Shortcut
Take a picture of the whole screen	Command (⌘)-Shift-3
Take a picture of part of the screen	Command (⌘)-Shift-4, and then drag the crosshair pointer to select the area. Continue to press the mouse button, release the keys, and then press Shift, Option, or the Space bar while you drag to resize the selection area. When you are ready to take a picture, release the mouse button. To cancel, press Escape before you release the mouse button.
Take a picture of a window or the menu bar	Command (⌘)-Shift-4, press the Space bar, move the camera pointer over the area to highlight it, and then click. To cancel, press Escape before you click.
Take a picture of a menu, including the title	Click the menu to display the menu commands, press Command (⌘)-Shift-4, and drag the crosshair pointer over the area. To cancel, press Escape before you click.
Take a picture of the menu without its title	Click the menu to display the menu commands, press Command (⌘)-Shift-4, press the Space Bar, move the camera pointer over the menu to highlight it, and then click. To cancel, press Escape before you click.

You can also take pictures of the screen using the Grab app.
Some apps, such as DVD Player, may not let you take pictures of the screen.

Source ;http://support.apple.com