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USM stands for Ultrasonic motor. It’s a type of motor used in Canon EOS lenses.
This is the motor which drives the autofocusing. Non-USM lenses often use an arc form drive (AFD) motor. This is good, but the USM is faster and quieter.
Some Canon lenses allow Full-time Manual focusing (FTM). This allows you to manually refocus the lens as soon as One-shot AF control is completed without switching the focus mode switch to manual focus. FTM is very useful if you need to fine-tune the focus in macro or super-telephoto photography.
Right from the early days of the EOS system, a new type of motor appeared in some EF lenses. The ultrasonic motor was first seen in the EF 300mm f2.8L USM lens, introduced with the first EOS cameras in 1987.
