The three things that control a photograph's exposure are shutter speed, lens aperture and ISO.
SHUTTER SPEED: The longer the camera’s shutter stays open the more light enters. To stop action, a fast shutter speed is needed; the exact shutter speed needed depends on the subject you’re shooting. To avoid blur, try to get your shutter speed fairly high; 1/500 should be the minimum and 1/1000 or above is even better. You may have to raise your ISO for the shutter speed you want to use.
APERTURE: Lens aperture, called an f/stop, is the size of the hole in the lens that light passes through. The larger the hole (smaller the f/stop number) the more light is passed and consequently the faster the shutter speed can be. Each lens has a maximum f/stop which limits how much light can pass through the lens. A "fast" lens has a very wide open f/stop (such as f/2.8 or f/1.4) and a slower lens may only be able to open to f/3.5. Some zoom lenses have a variable aperture which means the further you extend the zoom, the slower the lens becomes. [We’ll discuss this in more detail in a bit.]
ISO: The ISO setting controls the sensitivity of the camera to light. A higher ISO setting (ISO 1600 or 3200) is more sensitive to light and allows a faster shutter speed (or a lens with a slower maximum aperture) to be used.