Coils

Coils in electrical circuits are known as inductors. When electric current flows through an inductor, it creates a magnetic field that stores energy. If the current is interrupted, the field collapses which causes the voltage to increase sharply. It's that sharp increase that triggers a spark in the spark plugs, which in turn ignites the fuel in engine and starts the car.

Mechanical ignition

Older ignition systems used a mechanical distributor that actuated a switch, called breaker points. The points would open and cause the coil to induce the high voltage pulse while the rotating distributor would send it to the correct spark plug.

Electronic ignition

Later ignition systems rely on solid state electronics to control and distribute the high voltage pulse, but a coil is still required to produce it.

The Spark

The voltage spike created by the interruption of current in the coil reaches thousands of volts. This is sufficient to cause current to jump across the spark gap to ignite the fuel mixture.

Direct ignition

Some auto ignition systems do not have a separate coil but rely on individual coils built into the spark plugs to produce the high voltage. This is called direct ignition.

Source:

"Prentice Hall ASE Test Preparation Series: Engine Repair (A1)"; James D. Halderman; 2003.

"The Art of Electronics"; Paul Horowitz and Winfield Hill; 1983.

Autozone Reference Library: Ignition Systems