On a Mac, it is as follows:
If you want to print the full screen on the Mac then press the following keys."Shift" + "Cmd" + "3". You will then hear a camera shutter noise.
If you want to print a section of the screen on the Mac then press the following keys."Shift" + "Cmd" + "4". The mouse pointer will be replaced by a set of cross hair target. Use this to select the area for the screen grab you require. You will then hear a camera shutter noise.
If you want to print an open window of the screen on the Mac then press the following keys."Shift" + "Cmd" + "4" + "Space Bar". The mouse pointer will be replaced by an icon of a camera. Use this to select the window for the screen grab. You will then hear a camera shutter noise.
The resulting image file is usually saved to the Desktop. You can then edit it with your photo software and job done.