Aperture - the basics
To understand how aperture works on a camera it helps to compare it to the pupil of the human eye. The less light there is, the wider you open your pupil, while if there is a lot of light it narrows down to a small opening and blocks the amount of incoming light. The aperture on a camera, which is measured in f-stops, does exactly the same thing by controlling the amount of light that reaches the CCD. Lower f-stop numbers (e.g. 2.8) widen the aperture and allow more light to get to the CCD, while higher f-stop numbers (16 or 22) limit the amount of light by making the camera's aperture smaller. It should be noted that if the aperture is opened up by one stop, the amount of light reaching the CCD is doubled. At a given exposure value, aperture and shutter speed are always interdependent. If you change the former, the latter will have to be changed as well to keep exposure the same.
Aperture openings also control depth of field. Smaller aperture openings keep a larger part of the image in sharp focus while larger aperture openings will keep the subject or focal point pin-sharp while the rest of the image remains blurred. This effect is even more obvious with tele lenses as they have smaller depths of field than wide- angles.