The difference between a screen shot (command shift 3) and a screen grab is that a screen shot takes an instant photograph of whatever in on the screen, whereas a screen grab (command shift 4) allows you to choose the particular parts of your photograph by using arrows and dragging them to the corners of the particular object you want to keep in frame for the photograph. This also means that the photograph is not taken instantly because it needs to give you time to choose what the highlight with the arrows. A screenshot is helpful is you are going through a lengthy process and you want to take a quick photograph, so that at the end you can demonstrate the steps you have taken to reach your goal. A screengrab is more helpful if you want to show just one small part of the image. This may be because it is very important or that it may be because you have too many things on screen and you cannot see the particular part of the image that you are wanting. Once you have used command shift 3/4 then the photographs are stored on the computer like as if you would have uploaded photographs onto there.