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Three elements affect exposure: aperture, shutter speed and ISO. Aperture
Aperture is the size of one of the basic elements of a lens that controls the amount of light getting through and the amount of the scene that is in focus (Depth of Field). There's an element of every lens that controls the size of a circle that lets light through the lens onto the film or sensor.
Depth of Field
The aperture controls the Depth of Field. This is the amount of the scene that is in focus. This is the most important effect of aperture and its worth understanding because its a useful creative tool. Often portraits are shot with a shallow or thin depth of field so the subject (the person being shot) is in focus and the background is out of focus. Often landscape photographs are shot with a large depth of field to get everything pin sharp (the photographer's expression for "very sharp") from a rock in the foreground to the clouds in the background. Closeup shots of flowers want a medium depth of field for reasons I'll explain later.
F-numbers
Aperture is measured in f-numbers, like f2.8 (a shallow depth of field), f11 (a medium DoF) or f22 (a large DoF). The easy way to remember the relationship between aperture number and DoF is the smaller the f-number the less of the scene is in focus.
F-number's aren't measured like 1, 2, 3. The numbers used in camera are:
1.0 1.4 1.8 2.8 4 5.6 8 11 16 22 32
Each relates to a bigger DoF going from left to right (----->)
Each is also a step smaller aperture, i.e. the aperture is smaller and less light gets through. This won't affect the image quality because the camera will automatically adjust the shutter speed to let in more light.
Wiki has a useful page if you want to get into some of the science
A quick note on terminology
Between each f-stop is a "stop", so there's two stops between f1.0 and f1.8. One stop is half or twice as much light getting through, so there's a quarter less light getting through between f1.8 and f1.0 or four times as much light at f1.0 or two stops difference. This also applies to shutter speeds so there's two stops between 1/15 second and 1/90 of a second (which go up and down in a linear way). Half a stop is the difference between f6.3 and f8.
"Stopping down" is the jargon used for reducing the aperture or closing it down to get more DoF. Shooting "wide open" is the lens used at its widest aperture or smallest f-number (e.g. f2.8) to let in the most light for high shutter speeds in low light.
These are the basic. Read more once I write the creative aperture page.
