The aperture (also known as f-number) is an adjustable diaphragm of over overlapping blades which can be thought of like the iris of the eye, the aperture value represents a ratio of the equivalaent focal length of a lens to the diameter of its entrance pupil. And can be represented in many ways: f/8 F8 1:8. The smaller the f-number, the larger the aperture, f/2 means that the aperture is half the size of the lens, an aperture of f/11 means that the aperture is 1/11 the size of the lens. Aperture also has a direct effect on depth of field and also shutter speeds.

You will also note that lenses are also marked with their "maximum aperture", this represents the effective aperture that the glass within the lens causes to the light passing through it, your largest possible aperture when using this lens.

Lenses with a larger maximum aperture (eg. f/2) would be called fast (they allow more light through to the CCD), zoom lenses are normally quoted with two f-numbers (eg. f/2.4 - f/4.5) indicating the maximum apertures at its minimum and maximum focal length.