There are 3 elements that work harmoniously in determining exposure. Two of them, shutter speed and aperture, are functions of the camera's lens. Aperture refers to how large or how small the opening of the lens is when the shutter opens and shutter speed is how long that shutter keeps the blades in the lens open, allowing light to come in. The third element is ISO. Think of ISO as how sensitive to light the film or sensor is. Generally speaking, if a photographer will be shooting on a sunny day in manual he or she will select an ISO of 100 or so, and if he is doing a shot at night where he must use a long exposure (which requires the camera to be on a tripod) he would select an ISO of up to 3200.