Electrical stimulators typically come with a range of probes and attachments. Here we see a stimulator with a probe intended for vaginal or anal insertion
A penis wired up for electrical stimulation
As shown here, one form of electrical play has the submissive connected to one electrode of a stimulator and their dominant to the other. The dominant can then use their fingers or other parts of their body to complete the circuit and allow current to flow through their submissive, such as through their nipple
T.E.N.S. units (or Transcutaneous Electrical Nerve Stimulators) are battery-operated devices intended to trigger involuntary muscle contractions through the skin. They consist of electrodes which are taped to the skin and which are connected to a control box.
T.E.N.S. units are sometimes claimed to have medical uses but any benefits are unclear. In BDSM their main effect is to allow a dominant or top to control muscle movements of their partner. Sometimes one electrode is used internally (e.g., anus or vagina).
As opposed to violet wands---which only create sensations, T.E.N.S. devices actually cause feelings of involuntary control by the partner.