Transcutaneous electrical nerve stimulation (TENS) is a therapy that uses low-voltage electrical current for pain relief.
When the current is delivered, some people experience less pain. This may be because the electricity from the electrodes stimulates the nerves in an affected area and sends signals to the brain that block or “scramble” normal pain signals. Another theory is that the electrical stimulation of the nerves may help the body to produce natural painkillers called endorphins, which may block the perception of pain.
People use TENS to relieve pain for several different types of illnesses and conditions. They use it most often to treat muscle, joint, or bone problems that occur with illnesses such as osteoarthritis or fibromyalgia, or for conditions such as low back pain, neck pain, tendinitis, or bursitis. People have also used TENS to treat sudden (acute) pain, such as labor pain, and long-lasting (chronic) pain, such as cancer pain.
You can set the TENS machine for different wavelength frequencies, such as a steady flow of electrical current or a burst of electrical current, and for intensity of electrical current. Your physical therapist, acupuncturist, or doctor usually determines these settings.
The full overview is available here: http://www.webmd.com/pain-management/tc/transcutaneous-electrical-nerve-stimulation-tens-topic-overview