The term "f-stop" refers to the amount that the lens of a camera is open, the f stands for focal. F-stop is often used interchangeably with aperture and is giving you a measure of the size of the aperture ie. how much light is getting into the camera to strike the sensor.

The f-stop or aperture controls how much light gets through to the camera sensor, you can contrast the f-stop with the shutter speed. The f-stop controls how much lights gets through while the shutter speed affects how long the sensor is exposed to this amount of light. The aperture is also often compared to the pupil in your eye. You might recall that if there is very little light your pupil dilates to let in more light and that when you are in sunlight your pupil contracts so that you are not overwhelmed by the amount of light getting into your eye. The aperture of a camera or the f-stop functions in the same way, it controls how much light gets through.

f-stop or apertureCredit:

Hopefully the image above is helpful in giving you a good visual representation of the aperture in a camera and how it works.

Aperture settings or f-stops, are expressed as a ratio between the diameter of the aperture and the focal length of the lens. We won't delve into the intricacies of exactly how the f-stop is determined the simplest way to think of it is that each of these progressive f-stops lets in half as much light. The standard aperture values are:

f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32

You should note that the higher the number the lower the amount of light that is allowed in. This is so because the measurement we are using for the f-stop is a ratio. When someone says that an aperture is Open they are talking about the maximum setting on that particular lens, conversly when they talk about an aperture being Closed they are referring to the minimum setting.

Lenses are referred to by their maximum aperture and different lenses will often have different apertures. A cheaper lens may only open to f4.0, not letting in as much light as a more expensive lens that will open to f1.6. Lenses which have wide maximum apertures and let in lots of light are called fast lenses. Lenses which let in comparatively less light at their maximum apertures are called slow lenses.

Changes to aperture will effect "Field of Depth". These changes will also effect your images in other ways, for example the size of aperture used when taking night time scenes will give you different effects. If you use a smaller apature setting you can  enhance the way lights will look and get more of a radiating effect.