Lens autofocus systems can come in many forms, from basic screw-drive, through ultrasonic motors, stepping motors, and more recently, voice coil motors (VCM). What is VCM? To quote Canon, linear VCM autofocus:
"....uses magnetic force for fast, fluid-focusing adjustments. The focus unit is attached to a coil of wire which is positioned between drive magnets. These drive magnets shift back and forth to adjust the position of the magnetic field, which in turn moves the coil and the focus unit linearly in the lens barrel."
