They're not.
It's about the number(s) after the focal length. For example, your 18-55 kit lens is probably described as: Nikon 18-55mm f/3.5-5.6G AF-S DX VR
The
f/3.5-5.6 bit is the
maximum aperture of your lens--NOT the aperture range of the lens. This is the widest possible aperture setting you can use with this lens. Because your 18-55 is a zoom lens, it can have two maximum aperture numbers: one for the wide end of the zoom range, and one for the tele end. At 18mm, your max. aperture is f/3.5, at 55mm, your max. aperture is f/5.6.
With max. apertures, f/1.0-f/2.8 is considered "fast", f/4 is "medium" and f/5.6 and greater is "slow". The speed of the lens refers to how fast a shutter speed you can get with the lens, because of the size of the maximum aperture.
With aperture, the smaller the f-number, the
larger the aperture is. This is because it's a ratio. Just remember that the f stands for "focal length", and that it's written as f/# for a reason. So, just as 1/2 is larger than 1/8,
f/2 is larger than
f/8.
All lenses can be stopped down to smaller apertures, but not all lenses can be opened up to wider ones.