Exposure is the amount of light per unit area reaching a photographic film or electronic image sensor, as determined by shutter speed and lens aperture.
Perfect Exposure
Perfect exposure is when a picture has a perfect amount of light and the subject of the photo can clearly be seen.
Over Exposure
Over exposure is when a picture has too much light.
Under Exposure
Under Exposure is when a picture is too dark and the colour isn't there however some people over expose their picture to create silhouettes.