USM stands for Ultrasonic Motor, Canon’s name for an autofocus motor that uses ultrasonic vibration to drive focusing.

In practical terms, USM lenses are generally:

• faster to focus than older non-USM/micro-motor designs
• much quieter during autofocus
• often better for tracking moving subjects
• often capable of full-time manual focus override

Canon has used different USM implementations. Ring-type USM is generally the most desirable: it’s very fast, quiet, and typically allows full-time manual focus without switching AF off. Micro USM can still be an improvement over older basic motors, but features vary by lens, so not every Micro USM lens behaves the same way.

Why you’d want it:

• quiet AF, useful for wildlife and discreet shooting
• quicker autofocus response
• full-time manual focus on ring-type USM and many Micro USM lenses

Main downside:

• higher cost compared with simpler autofocus motor designs

So, in general, USM is a positive feature on Canon lenses. If you’re comparing lenses, ring-type USM is usually the version people prefer.