There are many fuses, each protecting a small group of components, so that one blown fuse does not shut down the whole system. Many of the fuses are grouped together in a fuse box, but there may also be line fuses in the wiring.
Electricity flows from a battery in one direction only, and some components work only if the flow through them is in the correct direction. This acceptance of a one-way flow is called polarity. On most cars the negative () battery terminal is earthed and the positive (+) one feeds the electrical system.
Where wires run side-by-side they are bound together in a bundle, in a plastic or fabric sheath, to keep them tidy and less difficult to fit. This bundle of wires stretches over the length of the car, with single wires or small groups of wires emerging where necessary, and is called the wiring loom.
The side and tail lights, however, which you may need to leave on when the car is parked, are always wired independently of the ignition switch. When fitting extra accessories, such as a rear window heater which consumes a heavy current, always wire it through the ignition switch.