TENS is an abbreviation for transcutaneous electrical nerve stimulators. A TENS machine is a device that makes use of electrotherapy to facilitate nerve stimulation and encourage therapeutic healing from acute or chronic pain.
On the other hand, EMS is an abbreviation for electronic muscle stimulators. EMS machines also use electrotherapy, but they're instead used to facilitate the contraction and relaxation of muscles. This article discusses the two types of machines (in question and answer format) for the benefit of affected patients.