manual focus is done using the focus ring on your lens, autofocus is done using your focus motor in your len's, camera or both. manual focus is used on camera's that don't have focus motors, or when you want to control the focus and not have it constantly changing. I use manual focus at races so cam does not keep trying to focus on the fence or other objects beside the one im trying to shoot. I autofocus to a point on the track that i am using as an aiming point, then i put camera on manual so focus does not change.