Take pictures of the screen on your Mac
Press Shift-Command-3 to take a picture of the whole screen. To capture just part of the screen, press Shift-Command-4.
To take a picture of a window, press Shift-Command-4-Space bar, move the pointer over the window, then click. To cancel, press Escape before you click.
After you take a screenshot, it appears as a PNG file on your desktop.