Aperture is the adjustable size hole that light travels through. The term is also synonymous with the size at which the aperture is set to.
The physical aperture of a camera is the mechanical system of blades in the lens that adjusts to various sizes. Lenses often have an aperture (or f-stop) specification, this indicates the maximum aperture the lens is capable of opening to.
Adjusting the aperture of your camera specifies the size to which the aperture will open to when taking a photo. This is generally referred to as the f-stop. The f-stop is the aperture relative to the focal length of the lens, and is usually written as a power of the square root of 2: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, etc, where f/1 is a larger (or wider) aperture than f/8.
Aperture affects the focus of an image. A wide open aperture will have a shorter depth of field, whereas a more closed aperture image will have a much longer distance of the scene in focus. Photographers use this to their advantage to selectively focus or blur subjects at different distances (see Composition).
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Low apertures (higher f-stop numbers) will require more time (Shutter Speed) and sensitivity (ISO) to achieve the same exposure as higher apertures (low f-stop numbers).