Some Canon lens terminology you need to know Ken.
USM is Ultra Silent Motor. Any lens with this feature has incredibly quiet autofocus focusing.
IS is image stabilisation. These lenses are expensive but enable you to hand hold the lens 2 or 3 stops slower in poor light than you would normally be able to and still get sharp (shake-free) results.
Any lens which ends in L is a top end Canon lens, guaranteeing great results at a price!
A new range of Canon lenses has started to appear designated EF-S These have the advantage of being designed as digital lenses from the outset, but they're only compatible so far with the 300D, 20D and the new 350D. They wont even fit any other Canon body. It's a bit of a worry committing to these lenses if you upgrade to a different body at some stage and can't fit them.