You can take a screenshot of just the active window by pressing Ctrl-Alt-PrntScrn. If you want everything, press Ctrl-PrntScrn.
On a Mac press Cmd + Shift + 3 it will create an image file of the screen shot on the desktop. If you want a screenshot of a specific area of the Mac screen press Cmd + Shift + 4 and then release all keys. The mouse cursor will change to a crosshairs, which you can move to the required area. Press down on the mouse button and drag over the area you want a screenshot of. Once you have selected the area let go of the mouse button and an image file will be created on the desktop.