TENS Machines - Transcutaneous Electrical Nerve Stimulation
They're generaly used for muscle stimulation as excercise for those who've lost control of muscle function. A company here was made apologise for misleading the public into believing they could turn you into Arnorld Swharttzenegger with one.
It's also used to ease pain. No one is certain how but many say it's of benefit. The unit about the size of a deck of cards, sends regular small pulses of electricity to pads you place on your skin with a conductive jelly. These little zaps of electricity stimulate the muscle and or confuses pain messages before they reach the brain.