USM - Ultrasonic motor
This is a big improvement over older micro-motor based autofocus systems, which are significantly slower and louder. There are a couple types of USM systems, the preferred type is "Ring Type" which allows for manual focus without turning off auto-focus.
The main benefits of USM:
- Faster focusing
- Quieter
- Full time manual focus (for ring-type USM)
Downsides:
- Higher Cost