Aperture is the opening of the lens, that lets the light through. In bright light it is restricted by the camera's exposure control mechanism,in low light it is opened up, in order to get just the right amount of light onto the film or digital sensor. It is expressed by numbers such as f2.8, f4, f5.6, f8, f11; the lower the number the bigger the aperture, thus a lens aperture of f2.8 lets through eight times as much light as an aperture of f8.
The Aperture is the number you get when you divide the focal length of the lens by the diameter of the aperture. The resulting number is called an (f/stop)
The smaller the aperture the larger the f/stop. A lens set a f/2.8 will let a whole lot more light through it at any given time frame than the same lens with its aperture set to f/22.
