Arc-form is what Canon calls the standard micromotor (non-USM) drive. It's generally an inefficient, whiny little motor that spins a physical geartrain which is usually made of plastic or nylon gears.
micro-USM (all USM lenses that do NOT have full-time manual focusing) use quieter motors to push a similar physical geartrain. Most of these are "nearly" silent, and certainly are compared with typical arc-form motors.
Ring-USM lenses support full-time manual focus and lack physical geartrains. They use magnetism to spin carriers and move elements. They're referred to as ring, because the magnets are arranged around the inner diameter of the lens barrel. The elements or carriers pass through the magnets.
Edit: I suspect that they're using a new form of motor which is much shallower than the typical arc motor. It'll be interesting to see just how silent and fast (or slow) it is.