Exposure refers to light reaching light sensitive material. It can be film, a digital sensor, or photo paper. Exposure determines how light or dark your image will be. It is controlled by three things. Aperture, Shutter Speed, and ISO (film speed).
Aperture controls the intensity of light passing through the camera lens. The size of the opening is given an f-stop value. The larger the number, the smaller the opening. Each change in f-stop doubles or halves the amount of light from the adjacent number. For instance, f4 allows twice the amount of light to pass through the lens as f2.8, and f4 allows half as much light to pass through the lens as f5.6. CLICK HERE FOR VIDEOS ON APERTURE
Shutter speed is the amount of time light is allowed to pass through the lens. It is measured in thousandth of a second. Typical shutter speeds are: 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15 1/8 1/4 1/2 1 second. The faster the shutter speed the more it will freeze motion. The same mathematical relationship exists like with apertures. Each change in shutter speed doubles or halves the amount of time light reaches the film. 1/60 is twice as fast as 1/30, but allows half as much light to reach the film. 1/30 is slower, but lets in twice as much light as 1/60.