Erotic electrostimulation (abbreviated e-stim  and also known as electrosex ) is a sexual practice involving the application of electrical stimulation to the nerves of the body, with particular emphasis on the genitals, using a power source (such as a TENS, EMS, Violet wand, or made-for-play units) for purposes of sexual stimulation. Electrostimulation has been associated with BDSM activities, and erotic electrostimulation is an evolution of that practice.
Electrostimulation, in general, can cause tissue damage or even death if misused. The most common problems arising from electrostimulation tend to be burns from lack of sufficiently wide surface contact, i.e. bad contact, between the electrode and the skin's surface.  Even at relatively low current and voltage, there is also risk of interference with normal heart function (potentially including cardiac arrest), and this risk is higher for those who use an artificial pacemaker or similar device or who have heart conditions.  Because of this, it is not advisable to place the electrical contacts in such a way that current passes through the chest cavity.  A few cases of accidental death as a result of autoerotic electrostimulation have been reported in the forensic science literature; the cases reported involved mains -powered, self-made devices, with current passing through the chest intentionally (usually via nipple stimulation) or unintentionally (for instance, touching an energized part with a hand). In one case reported in the press, a man from York, Pennsylvania was sentenced to 20-40 years in prison for third-degree murder and reckless endangerment after killing his wife with electrostimulation to her nipples directly from a power strip plugged into the mains.   
Newcomers are encouraged to research the hazards, limitations, and techniques of electrostimulation and the devices used.
The use of electricity for entertainment purposes dates back at least as early as the 1740s. In the 1830s, insertable electrode attachments for small magnetos could be purchased. Later in the 1800s, various electric belts (some complete with "suspensory sack") were advertised as cures for impotence. In the 1920s, the American Medical Association investigated such devices, and concluded that they provided "more or less mechanical masturbation". 
Modern electrostimulation first became recognized during the 1950s with the introduction of a device called the Relax-A-Cizor , which was originally designed to stimulate the muscles of a relaxing subject using electric currents as a means of "passive exercise ". Such power sources are still in medical use today and are known as EMS ( electrical muscle stimulation) units. Some people soon found alternative uses for the Relax-A-Cizors by placing the contacts on sexual parts of the body.
By the 1970s, medical TENS ( transcutaneous electrical nerve stimulation) units were also being used for electrostimulation. In the 1980s the first devices manufactured specifically for erotic electrostimulation became available, in particular the Titillator and the Pleasure Box, later known as the PES Power Box.
In the 1970s, experimenters noticed that bare speaker wires could deliver a jolt and began using recorded and live sound for electrostimulation. At that time, there were no professionally made attachments for such play, so people built their own out of copper plumbing parts and other metal pieces with attention to resistors placed in series with the human parts to control the current for safety. Although early e-stim units used only a simple, pulsed, sinusoidal wave, newer units use more complex wave forms and also allow for the use of ambient sound or prerecorded wave forms like music or specially designed computer files for specific types of stimulation. There are now sites dedicated to the creation of MP3 files specifically for erotic journeys or symphonies, which can include such routines as rewards, punishments, very strong, and pleasantly soft portions.