Canon AF Lenses, the diferences between types

Canon makes several different lens lines and grades.
First there is three grades of lenses Canon makes; EF, EF-USM and EF-USM L.
Then there is different "mounts" for film and digital cameras.
All EF mount lenses will fit all autofocus EOS film and digital cameras, however the EF-S lineup will only fit specific digital SLR (DSLR) cameras, those currently are the original Rebel Digital, The Rebel Digital XT and XTi, to EOS 20D and 30D. Any other Canon DSLR or Film SLR can be damaged by EF-S lenses being used as they extend deeper into the mirror gepartment to gepensate for the sensor size.
That said lets look at the differing grades of lenses.
EF Lenses are the least expensive of Canon's line up. Most of Canon's prime (or single focal length) lenses are going to be EF lenses and these are usually excellent lenses for the price. I personally thing everyone should own a 50mm lens and Canon's 50mm f1.8 EF lens is a astounding buy around US$90.00. Canon's EF Zooms ar usually going to have a plastic lens mount (easily broken I'm afraid to say) and lower quality lenses. Additionally the focusing motors in the EF lenses are smaller and slower than in the others. These lenses are going to be ok for someone who doesn't plan to make very big prints and wants to stay on a budget.

EF-USM Lenses utilize Canon's Ultrasomic Motors for their focusing system, these motors provid for faster focus and better durability over the life of the lens. Additionally most USM lenses will have metal lens mounts and better quality optics providing excellent performance for the hobbiest and advanced amature photograper. One drawback to these zooms is they are "variable aperture" lenses. This means that if you set the f-stop at the minimum setting at the widest focal length then zoom the lens the aperture will change as you zoom deeper into the lenses range. Making enlargements won't be too much trouble and the pictures will be sharper and clearer.
EF-USM L lenses provide all the same performance the EF-USM lenses do with the added bonus of using Canon's flourite glass, the very best glass Canon makes. Flourite lenses provide brighter and sharper images across the board and can show dramatic differences in prints as small as 8X10. These lenses are also usually faster in apertures oftentimes maintaining a f2.8 consistantly through the zoom. Unfortunately these lenses do tend to be bigger, heavier and more expensive than their USM cousins (often much more expensive).
One final word of warning, DON'T buy an L-Series lens, until you are ready to buy TWO. They make that much of a difference!