If you hold down the SHIFT and COMMAND key and then at the same time press the number 3, your mac will take a screenshot of your entire screen and save it as a .png (picture) file onto your desktop. Try it right now! Press SHIFT-COMMAND-3, you will hear a camera shutter style sound, and…
Did you know that if you hold down SHIFT-COMMAND-4 you can take a picture of any part of your screen?
How to do a screen casts on OSX.