Silent Wave Motor (SWM)

Conventional AF lenses:
These lenses use something called a coupling drive. The DC motor inside the body serves as the drive source. the body and lens are linked by gears, through which the force necessary to drive the autofocus mechanism are transmitted.

How does the SWM work?
The Silent Wave Motor (SWM), on the other hand, uses ultrasonic vibration to directly drive the lens.
 
"Ultrasonic" describes frequencies of mechanical vibrations that are inaudible to the human ear. When people converse, as you and I are, we can hear each other because the sound causes the air to vibrate. These vibrations spread out, or propagate, through the air, and are called "travelling waves".
In the SWM, ultrasonic travelling waves move in a spiral pattern inside the lens barrel. The motor is positioned on top of the waves, and they drive it from below.

Major advantages of the Silent Wave Motor
Just like the name says - the motor enables silent autofocus operation.
But it's more than just quiet - the mechanism drives the motor very smoothly and steadily, enabling highly precise control. Furthermore, the motor is in the shape of a cylinder, which allows it to fit nicely within the lens barrel.