Exposure is dependent on three things: ISO, Shutter Speed, Aperture
Aperture: Refers to the lens opening, and controls the intensity of light reaching the film/sensor. It is measured by fstop and directly controls the depth of field.
Shutter Speed: Shutter speed refers to the time the shutter is open to let in light. It controls the time. They are measured in fractions of a second.
ISO: (International Standards Organization) refers to the light sensitivity of your film/sensor. On an older film camera, ASA and ISO are interchangeable.
The higher the ISO number the more light sensitive the film/sensor will be. Lower ISO numbers are generally used during the day when there is alot of available light.