The term "USM" stands for "UltraSonic Motor" - a type of auto-focus drive fitted to many Canon lenses. Although the term USM is quite well known, less well known is the fact that there are actually two completely different types of USM - one good and the other probably not worth worrying about.
The original and best USM, known as a "ring type USM", was introduced by Canon in the early 1990's. This type doesn't even look like a motor and instead appears to be nothing more than two large rings with a few wires poking out of them. One ring is connected to the focusing parts of the lens and the other to the barrel, and when one part is made to vibrate at ultrasonic frequencies (hence the name) this translates into rotational movement of the other. This type is fast, almost completely silent (except to dogs and bats!) and there is no need for a gear train or other noisy mechanicals. Ring USM also has another advantage - full-time manual focussing or FTM in which, once auto-focus has completed, the lens focus can be "touched up" using the manual focus ring without having to switch the lens from AF to MF. Canon uses ring type USMs on nearly all its mid-range and L-series lenses except for a few very early designs which were introduced before USM was invented.
Ring type USMs:
The other (and lesser) type USM is known as a "micro-USM". This is a form of USM motor that Canon designed for their less expensive lenses so they can bill them as "ultrasonic" - essentially for marketing purposes and to cash in on the reputation of the ring-USM. These little motors operate on the USM principle but still use a noisy mechanical gear train and so are really not much different or better than their cheaper DC motor alternative. They also (except for the 50/1.4 and 28-105/4-5.6) lack FTM focussing and have a fairly poor record reliability-wise.
Micro USMs:
How can you tell which type is fitted to which lens? Well, as you might expect Canon deliberately do not distinguish between ring and micro-USM in the name so you have to look up the specs for each individual lens. Here is a list of all the USM lenses Canon have produced so far and the type used. (Note that many of these lenses also come in non-USM versions which use AFD or Micro-motor drives - these are not listed below. Also, not all of these lenses are available in all markets.)
Prime lenses:
EF14mm f/2.8L USM : Ring
EF20mm f/2.8 USM : Ring
EF24mm f/1.4L USM : Ring
EF28mm f/1.8 USM : Ring
EF35mm f/1.4L USM : Ring
EF50mm f/1.0L USM : Ring
EF50mm f/1.2L USM : Ring
EF50mm f/1.4 USM : Micro (with FTM)
EF-S 60mm f/2.8 Macro USM : Ring
EF85mm f/1.2L USM : Ring
EF85mm f/1.2L II USM : Ring
EF85mm f/1.8 USM : Ring
EF100mm f/2 USM : Ring
EF100mm f/2.8 Macro USM : Ring
EF135mm f/2L USM : Ring
EF180mm f/3.5L Macro USM : Ring
EF200mm f/1.8L USM : Ring
EF200mm f/2.8L USM : Ring
EF200mm f/2.8L II USM : Ring
EF300mm f/2.8L USM : Ring
EF300mm f/2.8L IS USM : Ring
EF300mm f/4L USM : Ring
EF300mm f/4L IS USM : Ring
EF400mm f/2.8L USM : Ring
EF400mm f/2.8L II USM : Ring
EF400mm f/2.8L IS USM : Ring
EF400mm f/4 DO IS USM : Ring
EF400mm f/5.6L USM : Ring
EF500mm f/4L IS USM : Ring
EF500mm f/4.5L USM : Ring
EF600mm f/4L USM : Ring
EF600mm f/4L IS USM : Ring
EF1200mm f/5.6L USM : Ring
Zoom lenses:
EF-S 10-22mm f/3.5-4.5 USM : Ring
EF16-35mm f/2.8L USM : Ring
EF17-35mm f/2.8L USM : Ring
EF17-40mm f/4L USM : Ring
EF-S 17-55mm f/2.8 IS USM : Ring
EF-S 17-85mm f/4-5.6 IS USM : Ring
EF-S 18-55mm f/3.5-5.6 USM : Micro
EF-S 18-55mm f/3.5-5.6 II USM : Micro
EF20-35mm f/3.5-4.5 USM : Ring
EF22-55mm f/4-5.6 USM : Micro
EF24-70mm f/2.8L USM : Ring
EF24-85mm f/3.5-4.5 USM : Ring
EF24-105mm f/4L IS USM : Ring
EF28-70mm f/2.8L USM : Ring
EF28-80mm f/2.8-4L USM : Ring
EF28-80mm f/3.5-5.6 USM : Ring
EF28-80mm f/3.5-5.6 II USM : Micro
EF28-80mm f/3.5-5.6 III USM : Micro
EF28-80mm f/3.5-5.6 IV USM : Micro
EF28-80mm f/3.5-5.6 V USM : Micro
EF28-90mm f/4-5.6 USM : Micro
EF28-90mm f/4-5.6 II USM : Micro
EF28-105mm f/3.5-4.5 USM : Ring
EF28-105mm f/3.5-4.5 II USM : Ring
EF28-105mm f/4-5.6 USM : Micro (with FTM)
EF28-135mm f/3.5-5.6 IS USM : Ring
EF28-200mm f/3.5-5.6 USM : Ring
EF28-300mm f/3.5-5.6L IS USM : Ring
EF35-80mm f/4-5.6 USM : Micro
EF35-105mm f/4.5-5.6 USM : Ring
EF35-135mm f/4-5.6 USM : Ring
EF35-350mm f/3.5-5.6L USM : Ring
EF55-200mm f/4.5-5.6 USM : Micro
EF55-200mm f/4.5-5.6 II USM : Micro
EF70-200mm f/2.8L USM : Ring
EF70-200mm f/2.8L IS USM : Ring
EF70-200mm f/4L USM : Ring
EF70-200mm f/4L IS USM : Ring
EF70-210mm f/3.5-4.5 USM : Ring
EF70-300mm f/4-5.6 IS USM : Micro
EF70-300mm f/4.5-5.6 DO IS USM : Ring
EF75-300mm f/4-5.6 USM : Micro
EF75-300mm f/4-5.6 IS USM : Micro
EF75-300mm f/4-5.6 II USM : Micro
EF75-300mm f/4-5.6 III USM : Micro
EF80-200mm f/4.5-5.6 USM : Micro
EF90-300mm f/4-5.6 USM : Micro
EF100-300mm f/4.5-5.6 USM : Ring
EF100-400mm f/4.5-5.6L IS USM : Ring
Finally, just because a lens has USM doesn't mean it has any better optical quality - it's just an AF mechanism 
Source: http://forums.dpreview.com/forums/read.asp?forum=1029&message=20104254
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