Photonotes.org has a really comprehensive Frequently Asked Questions for Canon EOS Beginners. If you are just buying your first D-SLR camera from Canon, or you are thinking of buying one, you will find a ton of information there.
This morning I was reading thru Part 3 that has to do with lenses. I got to one part that I thought would be useful to repost here. I’ve heard a lot of beginning photographers ask the question, “What are these numbers on my lens?”. Here is his answer:
Lenses have bits of text printed on the barrel or around the front element which convey a lot of important information about their properties. Be really careful when shopping for lenses. Two lenses may have very similar-looking names, but may actually be completely different one from the other.
Here are two examples. Note that all the technical terms used here are described later on in this FAQ.
CANON LENS EF 28-80mm 1:3.5-5.6. Ø58mm.
EF means that the lens is of the Canon EF type. EF lenses fit Canon EOS cameras and virtually no others.
28-80mm refers to the focal length of the lens. In this case there are two values since the lens is a zoom lens which can go from 28mm at its widest to 80mm at its longest. These numeric values in millimetres essentially indicate the coverage area of the lens.
1:3.5-5.6 refers to the widest aperture of which the lens is capable. The 1: is there since f/stops are, technically speaking, ratios. Since the lens is a zoom there are two aperture values - f/3.5 and f/5.6. This particular lens is an inexpensive amateur lens which can be opened up to f/3.5 at the widest end (28mm) but only f/5.6 at the longest end (80mm). This means it’s a fairly slow lens - it can’t let in much light, even its aperture diaphragm is when fully open.
Note that the autofocus motor type is not indicated, which means that the lens uses either an AFD (arc-form drive) or micromotor drive. Both types are slow and noisy compared to ultrasonic (USM) motors.
Ø58mm refers to the filter ring diameter. In other words, screw-on filters 58mm in diameter will fit this lens.
You can read more at Part III: Lenses.