Uses: Stun Guns are theoretically non-lethal weapons. They have 2 or 4 prongs protruding from the front. When those prongs make contact with a person, the person completes the circut, and they get shocked. Electricity follows the path of least resistance. Stun guns have been used as non-lethal weapons for quite a long time. Police departments have used them, along with electric cattle prods and tasers for riot control. Stun guns are a favored weapon of self defense for the private individual, because no license is needed, like that for hand guns. A purchaser must be over 18 to buy one. That is the only limitation.
Stun Guns are not Tasers. See Hexter's writeup for a very good explanation of a Taser. A taser is a projectile shock weapon. The purchaser must be over 18 and often needs a background check, depending on the state.
How the commercial models work: All stun guns work on the principle that high voltage, low amperage shocks cause muscle contractions. Anyone who has touched an electrical socket by accident, or licked a nine volt knows this, especially if they grabbed around a wire. It was probably hard to let go. The thing that stun guns do differently is that they rapidly turn the current on and off, causing muscle spasms. Most stun guns operate at between 100,000 and 650,000 volts.
Most commercial stun guns use two 9 volt batteries. There are two common designs. The first, older one, uses multiple capacitors, usually four. The nine volt batteries charge these, and they discharge in a sequence as the other ones are charging. These stun guns provide a stronger shock, at a rate of about 2 per second. They were not as effective, and could possibly do more cardiovascular damage.
The second type of comercial stun gun, such as the "Predator" uses one capacitor, and when someone pushes the main switch, the capacitor discharges about 15% of its charge, then recharges again within milliseconds, and releases again. About 80-100 shocks a second are provided with this design. Think Ultrasonic toothbrushes.
Effects: The actual effect of a stun gun is less then what it is made out to be. Notes from other people's personal experimentation (not on animals):
When one is shocked for 1-2 seconds, nothing happens except a light tingling in the area of contact. During longer periods of contact, the effects of course get worse.
Between 2 and 5 seconds, a lingering spasmatic twitch and tingling is caused around the place of contact. Because of the adrenaline rush caused by the muscle spasms, many people go into shock. This causes an effect similar to that shown in infomercials.
To cause shock and collapse, about 5-8 seconds of exposure is required, depending on a person's mass. No permanent damage is caused to otherwise healthy humans. I can still walk, move, and function as normally as I did before self-testing.