My Canon lenses have the designation USM on them, which I assume is for the same reason that they say "Ultrasonic". What does this mean, and why or why do I not want it?
USM - Ultrasonic motor (This is the Canon Terminology)
This is a big improvement over older micro-motor based autofocus systems, which are significantly slower and louder. There are two types of USM systems "Micro" and "Ring". The preferred type is "Ring Type" which always allows for manual focus without turning off auto-focus. Most, but not all, Micro USM lenses from Canon also have full time manual focusing.
Benefits of Ultrasonic motors:
USM is a Canon trademark, so similar terms are used by other manufacturers. These other names include:
Ultrasonic motor (USM) in this case should not be associated with human being audible frequency. I do not think the lens manufacturer builts it for the reason of no noise during operation, but the advanced simpler control of the lens movement by using the sinusoidal electric wave at the ultrasonic range frequency. Ultrasonic motor is low speed and high torque, and other than any of electric motors with high speed and low torque. Accordingly, such of focus hunting can be eliminated by the characteristic of USM and eventually we can obtain faster auto focus with.
Ultrasonic motor is anyway more expensive to be compared with any of electric motors.
The preferable kind of USM is ring type, which involving few rotating/moving parts and so the simple construction, so that resulting very fast performing autofocus, nearly no noise and we can rotate the manual focus ring at any time regardless the focus mode. Another construction is micro USM, which is working as well as conventional electric motor. Even so, the advance of low speed and high torque remains benefit for fast auto focus. As well as the conventional motor, some sprockets are required for connecting the motor and the frame of lens which therefore more mechanical constructions is required rather than the ring type. Consequently, we can hear a harder noise during the operation and little slower auto focus. The auto focus, still, much faster comparing to other type of electric motors.
As well as the system construction is nearly same with electric motor, the manual focus ring will be rotated as well when it is set under auto focus mode. The only switching to manual focus mode allows us to rotate the ring manually. The exception is EF 50 f/4 (micro) USM. You can rotate the focus ring manually at any time regardless the focus mode. A complex sprockets arrangement (look alike the latest car with automatic transmission, the wheel can be rotated at any direction under the engaged gear without damaging the gear box) leads us to operate the lens as well as by using the ring type.
I wish I can contribute to the discussion.
As many people have mentioned USM = Ultrasonic motor. Its a small motor built into the lens. This means that the camera will focus MUCH faster without making noise (or much noise). There are actually 2 kinds of USM, "Ring Ultrasonic" (good) and "Micromotor Ultrasonic" (not so good) see here. If you're interested in a more technical explanation form canon, see here.
Amazon shows a 50mm lens at $100 for the non USM and $300 for the USM. AFAIK its the same glass in in both, so you're paying for the quick and quite motor. Its also heavier.
You want it for a few reason:
You may NOT want it because:
Call me dumb but after few minutes of hard-thinking I still cannot come up with ANY other reason you may NOT want it. Making my answer almost identical to the previous one lol.