Both A.C and D.C. motors have similar functions. Both of them work by converting electric energy into mechanical energy. However, A.C. and D.C. motor are powered and controlled differently. A.C. motor is powered by alternating current. The stator in the electric motor has coils that supply alternating current.
The rotor rotates inside the electric motor coil to produce torque by the rotating magnetic field. A.C. motors require any equipment to start. D.C. motor is powered from direct current. Example of a powered source is batteries. The motor is usually externally controlled. The motors are self-starting.
USM and IS are two different things, but they are both used in camera lenses. USM is the acronym for Ultrasonic Motor. USM works by moving the lens element inside a camera so as to appropriately bring the subject into focus. This type of technology is used in capturing and making the faster autofocus of subjects that are on the motion. And the mechanism of USM helps in capturing some specific moments, that ordinarily cannot be captured when using common motors. IS, on the other hand, is the acronym for Image stabilization.
This works by reducing or by completely eliminating the amount of possible blurriness that might be caused due to the instability of the hands of the person using the camera, or the fast motion of the camera being used. IS is mostly needed, especially when you are shooting an object that requires zooming. IS is needed to reduce the blurring so as to have a clean shot.