Electro-acupuncture, sometimes called P.E.N.S. (Percutaneous Electrical Nerve Stimulation), is where a mild electric current is pulsed through the needles as a means of stimulation. The fact that precise voltage and frequency can be recorded and reproduced means it is popular in research trials.

It is a very modern addition to acupuncture but its origins go back to ancient Egypt and Rome where electric fish were used for pain.
It is still primarily used for pain conditions but also for a few other specified disorders where research is strong. I tend to think of it as a stronger kind of stimulation and reserve its use for cases with more pronounced symptoms of pain, returning to regular acupuncture or massage once a reasonable balance has been reached.