It's a basic guide to dSLR lenses, and describes the main features of a lens (focal length, maximum aperture, prime vs. zoom, stabilization and other goodies) in practical shooting terms. This way, you should be able to get a decent picture of what all the numbers stand for.
The second thing is, you are now going to have two camera bodies: the 350D and the 550D. You can put an 18-something on one, and a telephoto on the other, and have the best of both worlds.
If you're shooting outside in sunshine most of the time, you can probably get by with an f/4 70-200. The f/2.8s differ from the f/4s not only in max. aperture and price, but also in size and weight. The 70-200 is a white L. It's all-metal. It's bigger and heavier than any of the lenses you've got. The image quality is terrific, but you're going to want to get your hands on one to see if it's for you, first. It's an entirely different kind of lens than the ones you're used to. You may want to consider renting before you buy to see if it's for you.
Also, there are five different models. You have a choice of f/4 or f/2.8, with IS or without, and the 70-200 f/2.8L IS USM comes in Mark I and Mark II (the Mark II now brings the wide-open sharpness up to a par with the 70-200 f/4L IS USM's). Keep this in mind when you read older reviews before the f/2.8 IS II was introduced.
In specific Canon terminology, a lens is described as
EF, EF-S (and there are also two exotic designations, TS-E and MP-E)
EF = Electronic Focus (i.e., the Canon EOS mount)
EF-S = Electronic Focus - Small (i.e., the digital Canon EOS mount--the smaller sensor in the 1.6x crop bodies allows a lens with a smaller image circle. These lenses can only be used on 1.6x crop bodies, and not on the 1-series or 5D or film EOS cameras).
TS-E = Tilt Shift Electronic (manual focus tilt-shift lenses)
MP-E = Macro Photo - Electronic (the super 5x macro lens)
The focal length.
The maximum aperture.
L = "Luxury" / "Low dispersion" glass. The lens has special flourite or UD (ultralow dispersion) glass elements in it to help control chromatic aberration and increase light transmission. You tend to get better color, contrast, and chromatic aberration control with L lenses, as well as more usability features than non-L lenses. Also, it's rare for one to cost less than four figures.
USM = Ultra-Sonic Motor. A special focus motor that's silent and faster than non-USM motors. There are two types: ring-type and micromotor. Ring-type is the more desirable as it allows for full-time manual focus (i.e., you don't have to flip the MF/AF switch to move the manual focus ring). USM is particularly useful in fast-action photography at getting the lens to lock focus faster.
IS = Image Stabilization. The lens can move elements inside it to compensate for camera shake. IS is typically rated in stops. The older the design of the lens, the less the IS will compensate. IS is good for using slower shutter speeds and eliminating camera shake blur (particularly with longer lenses). But it does nothing for eliminating subject motion blur, because you still have to use that slower shutter speed. And a monopod can pretty much do everything IS does, stabilization-wise.
[roman numerals] the version of the lens. Most lenses don't have this, but a few do, and each version usually marks some form of improvement over the previous generation. The big exception to this would be the EF 50mm f/1.8 II, where the improvement was to make it a lot cheaper.
Then come the exotic designations: DO (diffractive optics) & Macro. DO is only found on two telephoto lenses, and it was a way of designing the optics to make a more compact lens. It's expensive and meant for folks trekking gear over African veldt or Arctic tundra. Macro means the lens has close-focus capability.
For macro, the two mid-range Canon choices are the EF-S 60mm f/2.8 USM Macro, and the EF 100mm f/2.8 USM Macro (the non-IS, non-L version). The 100mm is the more "general purpose" lens--it gives you more working distance so you won't scare off bugs or lizards, etc. The 60mm is crop-body only, a little sharper than the 100mm, but you have to work closer to your subject, so there's more work avoiding your own shadow, and it's really only good for tabletop objects or flowers--stuff that won't run when it sees you looming over it. It is, however, about half the size and weight of the 100 Macro.
Two places to go to get some opinions of lenses would be the-digital-picture.com (which is sort of the ultimate Canon geek's reviews of Canon gear), and the Fred Miranda lens reviews, where people dogpile on and you can get a lot of various views of the lens from different shooters.
Source
Hope that helped
