AFD - Arc-Form Drive, slowest and noisiest drive. No manual focus override. Bad for video.
MM - Micro motor, smaller focus drive, also noisy and without focus override. Bad for video.
STM - Stepper Motor, good for slow focus changes, quite silent, good for video.
USM - Ultrasonic Motor, fast for still photos focusing, almost silent. Not very good for video.
Nano USM - latest version of ultrasonic motor, made to be almost silent and good for video.