Unlike most camera makers Canon chose to position the autofocus motor inside the lens barrel rather than in the camera body when they designed the EOS system. This was arguably a wise move, since it means autofocus motor can be tailored to the requirements of each lens. A big telephoto lens can have a large motor and a small normal lens can have a more compact motor. By contrast, systems which rely on the autofocus motor being only in the camera body don’t have this flexibility - the motor is always the same regardless of the lens used unless you change camera bodies.

So. Canon employ a number of different motor technologies in their lenses. The first two types are never identified specifically on the exterior of the lens. You have to look up in Canon’s product literature to see which type of motor a given lens uses.

Traditional electromagnetic motor drives.
Such motors contain tiny wound coils of wire and rely on electromagnetic principles to turn a shaft. Little cogwheels and gears are then used to translate this rotational motion into the movement needed to adjust lens focus.

Arc-form drive (AFD).
Generally used in a number of older lower-cost lenses, AFD motors are simply little electric motors which drive a geartrain. They’re somewhat noisy - electric buzzing and grinding of gears - and not terribly fast. This isn’t a big deal on smaller lenses since the distances the motors must move the focussing elements aren’t very far. However, telephoto lenses with AFD motors can be quite sluggish.

Micromotor (MM) drive.
Generally used on a few older lower-cost lenses. Similar to AFD - slow and noisy and based around an electric motor driving a geartrain. Some particularly low-cost lenses use micromotor drives with rubber belts.

Ultrasonic motors.
Ultrasonic motors do not rely on magnetic coils like most electric motors. Instead they use extremely high-frequency vibrations which translate into circular motion. The result is a very fast and pretty well silent (to human ears, anyway) lens motor. There are two basic types employed by Canon.

Ring ultrasonic (USM) drive.
The kind you want. These motors consist of two metal rings which vibrate at a very high frequency. (have a look here for photos of these rings) Ring ultrasonic lenses are great because they focus quickly and silently and also support full-time manual (FTM). There are actually two variants of this design - see the FTM section below.

Micromotor ultrasonic (USM) drive.
This kind is less desirable. It’s a form of USM motor that Canon designed for their cheapie lenses so they can bill them as ultrasonic for marketing purposes. An MM lens replaces the standard magnetic motor with an ultrasonic motor but retains the usual geartrain setup. Such lenses are still reasonably quiet, though not as quiet as ring ultrasonic motors, but usually lack FTM - see below.

Note that, while all lenses with “USM” in the name contain an ultrasonic motor, Canon do not distinguish between ring and micromotor USM drives in the name - you have to look up the specs for the individual lens to find that out. Also, most non-L lenses with USM drives have striped gold lines painted around the end of the barrel. However, all L lenses have red lines painted around the end, whether or not they use USM (ie: Canon never have two painted rings around their lens barrels, and the red L line takes priority over the gold USM line).