Electricity can be dangerous as well as useful. The biggest risk is that you could get a shock, which can prove to be fatal. Electrical current flowing through your body heats up the body tissues it passes through for the same reason that a light-bulb filament heats up when electricity passes through. There used to be hot-dog cookers which operated on this principle (but the ends were charred and tasted very bad). This heat can burn the tissues, killing them. Electrical current passing through living tissue can also interfere with the electrical functioning of the nervous and muscular systems. Electrocution may result in instant heart stoppage, for example. Household electricity at 110 volts is high enough voltage to cause big problems when it flows through people's bodies. Even low voltages can cause lots of damage if enough current flows.
High-voltage electricity, when it makes a spark, can damage equipment. It may char the contacts, make little pits in the metal, blow fuses, burn wires in transformers, and otherwise make electrical and electronic equipment fail to work. Sensitive electronics, such as computer parts, are particularly sensitive to even tiny discharges of high-voltage electricity -- they can stop working and need replacement if touched by someone with a high static electric potential, often gotten by scuffing shoes on carpeting, or even just getting up out of a cloth chair. You shouldn't worry about electrocution when you experience a discharge of static electricity. It turns out that your capacitance is very small (maybe a hundred picofarads or so), and so the total charge accumulated on your body is tiny, even at high voltages. Discharging a small amount of charge reduces the voltage rapidly to zero. It is electrical current that causes damage, and when the source of high voltage can also supply a lot of current, that is the most dangerous situation. High voltage is worse than low voltage mainly because of Ohm's law -- with higher applied volgages, the current will be higher, until the charge runs out of course.
A recent warning was sent out about the possibility of starting a fire or even an explosion at a gas station (whoops -- you're in England -- the petrol station) because of static electrical discharges. If you are filling up the tank of your car, and for some reason go back and sit down on the nice cloth seats, get up again (acquiring a static electrical charge), and go back to the filling nozzle, the spark you make may ignite the gas fumes and cause an explosion. Beware!
On a bigger scale, a high voltage discharge caused by a short circuit from a high-voltage component to wires normally carrying low-voltage signals is the most plausible explanation for the ignition of fuel which caused the explosion of TWA flight 800 in 1996.
(republished on 07/22/06)