TENS info-
http://www.drugs.com/cg/how-to-use-a-tens-unit.html

TENS is a method of electrical stimulation which primarily aims to provide a degree of symptomatic pain relief by exciting sensory nerves and thereby stimulating either the pain gate mechanism and/or the opioid system.]
http://www.electrotherapy.org/modalities/tens.htm

This page you can find info on many of the various therapies.
http://www.electrotherapy.org/modalities.htm