What do the numbers on a camera lens mean i.e 28-55mm?

Perfect answer, teef.
But I think I'm going to try to add in a bit more. the 28-55mm numbers are not the only numbers on a lens.

If you see a f/, then a number after it, that's called the f-stop, or aperture. It tells you the minimum aperture rating. That means the narrowest depth of field, a useful feature for portrait photography to keep the background out of focus and your subject in focus, and the maximum iris size for more light and faster shutter speeds. When you see a number like 1:, followed by another number, that's the same thing.
There will only be one f-stop number listed on 'prime' (non-zooming lenses). There are usually 2 f-stop numbers listed on zoom lenses. That's because the minimum f-stop is usually different at different focal lengths. but, if there is only one f-stop number listed on a zoom lens, then that lens has been designed to keep the same f-stop, regardless of focal length. That's a very useful feature. It means that you can expect consistent results, even if you are varying your focal length between different shots.

Also, you will sometimes see another number with the (mm) sign, usually with what looks like a O with a line through it, like a Do Not Enter symbol. This is the filter diameter size for that particular lens. It's completely separate from the focal lengths of the lens. That's an important piece of information to have before you by any filter mount accesories (like lens caps, filters, hoods, etcetera.)

Oh, and if you have a 7 digit (or so) number near the base of your lens, that's probably the serial number....
Okay, now I'm just being silly. ;-D

its the focal length of the lens. the smaller teh number the wider the lens, 24-28-35mm are wide angle 50-55 are standard and 75-300 are telephoto.. they are fixed (135mm) or they zoom as in your example 28 to 55mm

The range of the lens. 28mm is fairly wide angle so you'll be able to see a lot through the lens and in your picture, at 55mm things will look closer but you will see less of the overall picture. The bigger the number the closer things will look.

The above answers are correct, it's the focal length range of the lens (in this case it is a zoom lens). Most compact zoom cameras (film anyway) tend to be around 28mm ~ 80mm which is roughly a 4X zoom.

With digital though the focal lengths are entirely different, especially with the SLRs and different sized sensors. 18mm is then the equivalent of a 35mm film focal length.

Moving into SLR photography you need to forget about the magic zoom factor and concentrate on focal lengths.

A focal length of 50mm is regarded as "Normal" and all other references are drawn from here. This is something supposedly similar to what the human eye sees. Normal focal lengths are used for portraits, although up to 85mm is still considered a portrait lens.

Any number lower than 50mm is considered a wide angle lens, and any number over 85mm is considered telephoto. SLR lenses can have fixed focal lengths, or they may zoom. An 18-55mm lens is wide angle to normal zoom lens. A 70-200mm lens is a zoom telephoto lens. Notice how a lens can be a zoom lens but not necessarily a telephoto lens, this is a popular misconception.

teef and devil dog got it covered I am just a points whore....