USM Technology from Canon.

The USM,Ultrasonic Motor, is a driving motor that Canon commercialized for the first time on camera lens in the world. Ultrasonic motors, are based on a different principle in which rotational force is producing from ultrasonic vibration energy. USM makes focusing very faster, high accurate and quieter than ordinary focusing drive motor and speed-reducing gear train.

USM construction is very very simple, consisting of an elastic stator and a rotating rotor. The stator with a piezoelectric ceramic type elements in that, while energized by an AC voltage, it vibrates and to generate ultrasonic vibration energy. That vibration energy is using to produce continuously rotate the rotor through the pressure contact between the rotor and stator.