This video is not only for nikon users, basic concept is similar in all cameras include canon sony etc.
AF-S (AF single, sometimes called single Time AF) mode, is good for photographing subjects that don’t move, such as flowers or portraits etc. It locks the focus on the non moving object that you want to photograph. You can then recompose the shot and take the photograph.
AF-F (AF continuous, sometimes called Full Time servo) is good use when photographing moving objects. When your camera is set to AF-C and you focus on a moving subject, for example a dog running towards you, the focus will stay on the animal so long as your shutter button is held half way down. In other words, the camera will keep re-focusing as the animal moves. That is, so long as you keep your shutter button held half way down.