Canon make telephoto zooms which fall mostly into two basic categories - very cheap slow lenses with mediocre optical quality and very expensive fast lenses with fabulous optical quality. There are few intermediate choices, though the 70-300 4.5-5.6 IS USM is one to consider. Canon have also never built any affordable long EF telephoto prime lenses - all EF telephoto primes longer than 135mm are L series lenses.

There are a number of Canon zoom lenses in the 75mm to 300mm range, however. Here’s a bit of basic information about some of them.

75-300 4-5.6
75-300 4-5.6 USM
75-300 4-5.6 II
75-300 4-5.6 II USM
75-300 4-5.6 III
75-300 4-5.6 III USM
Canon sell and have sold quite a few different lenses with focal lengths from 75mm to 300mm. All apparently use the same optics - they just have different cosmetic styling of the lens barrel (the mark III version, for example, has a silver ring on the end to impress novice camera buyers) and different autofocus motors. They’re very inexpensive lenses as telephoto zooms go, but offer fairly mediocre optical quality. At the short (75mm) end they’re not too bad but at 200-300mm they tend to get very soft (slightly blurry). You really need to stop them down to f/8 or f/11 or so for okay optical quality, which of course requires longer shutter speeds.
The 75-300 USM models all have micromotor USM autofocus mechanism which do not support full-time manual (FTM). All other versions of this lens use slow and noisy micromotor or DC motor drives. These lenses, while in the “cheapie” category of Canon lenses, have metal lens mounts. They do not, however, have distance scales. All versions of this lens have rotating ends when focussing, which makes using a polarizing filter rather inconvenient.

75-300 4-5.6 IS USM
One notable lens in the 75-300 series is the IS model, which offers image stabilization for improved shooting at lower shutter speeds. It was the first IS lens offered by Canon, but the lens sadly has the same unremarkable optics as the other 75-300 lenses.

70-300 4.5-5.6 IS USM
Do not confuse this lens with the cheaper 75-300 crowd or the more expensive DO lens with almost the same name. This lens combines decent optics with image stabilizing and is an excellent compromise for the advanced amateur. It’s not quite as sharp as the 100-300 5.6L, but IS makes it a lot more convenient since you end up with a much higher percentage of keepers when used off-tripod.

70-300 4.5-5.6 DO IS USM
This lens is particularly unusual in that it’s the first zoom lens to use diffractive (DO) lens elements. This Canon technology allows for smaller and lighter lenses. The 70-300 DO is considerably shorter than its 75-300 siblings, and has image stabilization (IS) technology to boot. It isn’t cheap, however. Do not confuse it with its non-DO sibling.

90-300 4-5.6 USM
I really don’t see the point of this lens. It appears to be pretty well the same as all the cheap 75-300 lenses, only its short end starts at 90mm. It’s USM but only micromotor USM and thus has no FTM.

100-300 4.5-5.6 USM
This lens is, in terms of build quality and physical appearance, the telephoto zoom counterpart of the 28-105 3.5-4.5 USM and the 24-85 3.5-4.5 USM. It has reasonably solid construction, a fast and silent ring USM autofocus drive which supports full-time manual (FTM), does not having a rotating end and has a distance scale. Optically the 100-300 USM I had was very slightly sharper than the 75-300 at the long end, but many people report that there’s really no difference between their samples of the lenses. Essentially the 100-300 USM gives you improved focussing speed and a more convenient user interface compared to the 75-300s, but not improved optics. You also lose 25mm and half a stop off the short end, for what it’s worth.

70-210/3.5-4.5 USM
The predecessor to the 100-300 4.5-5.6 USM. Very similar in size and construction, only with a focal range that’s shorter at each end. Offers similar optical quality.

100-300 5.6L
An older and now discontinued lens, the 100-300 5.6L is an interesting lens in that, while it’s technically an L series lens with fluorite and UD lens elements, it doesn’t have the tough build quality of a typical L series lens sold today. It uses a slow and noisy AFD autofocus motor and has an awkward (low-profile) and fiddly (hard to slide) MF/AF switch. The manual focus ring is also rather gritty to turn. However, it does offer considerably improved optical quality over the 75-300 series and the 100-300 USM. It's also sharper than the 70-300 IS USM. So if you can deal with the slow optical speed (maximum aperture of only 5.6) and the horribly sluggish autofocus motor it’s well worth looking into if you’re on a tight budget.

50-200/3.5-4.5 L
Similar to the 100-300 5.6L, in that it’s a first generation EF-mount L series lens. It has the optics of a modern L series lens - but not the build quality - and has a push-pull zoom design. The 50-200 isn’t a bad lens, but for some reason holds its used market value rather well. Personally I think the 70-200 4L USM is a better deal. It’s typically not much more money but gives you better build quality and silent focussing USM.

70-200 4L USM
This lens, the smaller and cheaper sibling of the impressive 70-200 2.8L USM professional lens, is considered a bargain by many EOS users. It costs three times as much as the cheapie lenses but it’s sturdy, focusses quickly with a ring USM system with FTM and, most importantly, it has great optical quality. It’s bigger and heavier than the consumer lenses, but if you want something good but can’t afford the 2.8L, consider this lens. It doesn’t use a huge 77mm filter like the 2.8L - it uses a 67mm filter like the 24-85 3.5-4.5 USM. This is a little unfortunate, has hardly any other Canon lenses have 67mm filters.

70-200 4L IS USM
The image stabilized sibling to the 4L. Visually almost identical, down to the 67mm filter, but much much more expensive than its non-L counterpart. According to some reviewers, one of the sharpest Canon lenses in its whole zoom lens range.

80-200 4.5-5.6
80-200 4.5-5.6 USM
80-200 4.5-5.6 II
Plastic cheapies, basically analogous to the 28-80 plastic cheapies. They are very lightweight and portable, though. If you want something really inexpensive and never enlarge your photos past postcard size then they’re fine.

100-300 5.6
With pretty well identical build quality to the 100-300 5.6L, this older lens has all the disadvantages of the 5.6L without the sharp optical quality of the L lens.