The aperture in Photography is the iris of the lens. The size of the aperture determines how much light comes through the lens and can be balanced with how long the lens is open to make an exposure. The aperture controls depth of field, that is the depth of space that is in focus. The smaller the aperture the deeper this space is.
A photographer may want to control the depth of field using the aperture setting to make sure the background is or is not in focus. Selective focus can be a powerful tool in a photo. If there are subjects in the composition at differing distances a greater depth of field may be required to insure they are all in focus together. An example of this might be a large group of people for instance.
The depth of focus at different f/stops will vary per the focal lens and design of different lenses. In general it will be greater at a given aperture with a wider lens and shorter with a longer (telephoto) lens. Also the closer your subject is the the camera the narrower the depth of focus as the depth increases with distance. An extreme example is f/8 for a given lens may produce a focal depth of 2ft for a subject 10' from the camera, this may be only 4" when the subject is 2' from the camera and may be infinite (50'-infinity) when the subject is 60' from the camera. This property can be used in the design off your photo for practical or artistic purpose.
Because Aperture, shutter speed and ISO are tied together in determining an exposure changing the aperture may require also changing one of both of the other with some trade-offs such as a slower shutter speed which may result in blur due to movement (by the camera and/or the subject) or increased noise is the ISO needs to be raised significantly.
09/24/12 John Lacy, Proshooter.com
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