Exposure

Definition: Exposure is the amount of light collected by the sensor in the camera during a single photo. If the shot is exposed too long the photograph will be washed out. If the shot is exposed too short the photograph will appear too dark.
Aperture: the size of the lens opening that lets light into the camera
Shutter Speed: the amount of time the sensor is exposed to light
Metering: the brains behind how your camera determines the shutter speed and aperture, based on lighting conditions and ISO speed.

References:
http://www.photonhead.com/simcam/shutteraperture.php
http://www.picturecorrect.com/photographytips/ExposureBasics.htm

F-stop: