A quick search gave me an excellent solution to the problem here. Here you go:
Apple (Command) Key +Shift+3
Captures entire desktop to a file on the desktop as 'picture #' . This option lets you capture the whole screen. If you want just one window on your screen, you will have to edit the picture using image editing software.
Apple (Command) Key +Shift+4
Allows you to use your mouse to select a specific part of your desktop for capture. This will turn your mouse pointer into a cross, please hold down the mouse button and drag to select the part of the screen you want. When you release the button the screenshot will "snap" that part of the screen. Press 'Esc' to release.
Apple (Command) Key +Shift+4 then press Spacebar
Allows you to select which window to capture.
Even better than on my PC!