The hole which lets light in.
f-stop is the measurement of the size of the aperture and controls brightness. Aperture is measured in "stops", which are doubling or halving units.
- f/5.6 is double of f/8, and quadruple of f/11
- f/8 is half of 5.6, and double of f/11
f/11 is half of f/18, and a quarter of f/5.6
A large aperture gives a softer background but keeps the foreground sharp. A small aperture keeps more of the background sharp.
For a portrait, a larger aperture is useful to soften the background and sharpen the subject.
For a landscape, a smaller aperture is useful to keep everything consistently focused.
How far one can open the aperture is referred to as its "speed". A larger aperture allows much more light to reach the "film" and therefore reduces the exposure time of the film. So the larger the hole, the more light and the faster the exposure.
Aperture changes with zoom lenses. Zooming in will shorten the aperture.
Faster aperture (a wider hole) is better in low-light, to allow more light in, or for fast-moving subjects.
The speed of the shutter, the device which allows light in, also influences things. A faster shutter speed gives less opportunity for light to enter, and a slower shutter speed gives more opportunity for light to enter.