Control how much light is passed through the lens. Every step in this table represents a ONE STOP change in light. Like the pupil in a human eye, the aperture on a camera controls light. It does so by closing up to restrict light, and opening up to let it through. Okay, so now you know all about the shutter and the aperture.
Exposure is about different combinations of shutter and f-stop settings.
These combinations can drastically affect the finished picture. For example, the following three pictures have been given an equal amount of light, but the f-stop and shutter combinations make each one unique. Check out the SimCam. Links to more in-depth photo info.