The aperture, or iris on a camera, controls the amount of light entering the lens. Some point and shoot cameras have fixed apertures that do not move, but more sophisticated cameras will have a movable aperture that creates a wider or narrower hole through which the light passes.
The primary function of the aperture is to control light. By increasing or decreasing the amount of light entering the camera, it controls the exposure of the picture, making it lighter or darker. Secondary functions of the aperture are to control depth of field and impact the shutter speed.
The aperture is a circular device in the lens that works much like the human iris. It is composed of a series of triangular flanges that fit together to leave a round hole in the middle. By rotating a dial on the outside of the lens, the flanges can be pushed in or pulled back, creating a smaller or larger hole.
Aperture openings are traditionally measured in f-stops. Confusingly, the smaller f-stops correlate with larger openings, and vice versa. A very small f-stop such as 2.8 will mean an almost fully open aperture; a large f-stop such as 16 will be very small.
Depth of Field
The aperture controls the depth of field, which meaning the depth of the shot that will be in focus. For instance, with a very wide open aperture, the depth of field is a thin slice, meaning the object you are focusing on directly will be in focus, but something a foot or so behind it will not. With a smaller aperture, the depth of field is quite broad, and something 10 feet away can be just as sharp as something 50 feet away.
By controlling the light going through the lens the aperture controls the exposure, in tandem with the shutter speed and ISO value (which is a measure of the sensitivity of the film or sensor). Closing the aperture requires a slower shutter speed. This can be desirable for certain effects, such as a blur to imply speed. A high ISO value means less light is necessary for proper exposure, and a smaller aperture can be used, yielding a higher depth of field.