The purpose of the ignition system is to create a high voltage electrical spark, much like a bolt of lightning, which is directed into the individual cylinders in order to ignite the compressed air and fuel mixture inside. The ignition system is divided into two parts, the primary ignition system and the secondary ignition system.
The primary ignition system uses the 12 volts from the vehicle's electrical system to supply power to the primary windings of the ignition coil. The ignition coil is simply an electrical transformer that converts the 12 volts in its primary windings to the much higher (secondary) voltage generated in its secondary windings. This secondary voltage can be up to 80,000 volts.
The secondary ignition system is comprised of all the components that carry the higher, secondary voltage. These components can very greatly depending on the specific vehicle. For many years, the secondary ignition system used a distributor to transmit the high voltage current from the ignition coil to each spark plug via the ignition wires. As electronics began taking over, the distributor was eliminated and more ignition coils were added. Modern engines now have an ignition coil for each cylinder, which is mounted directly on top of the spark plug, eliminating the need for spark plug wires.
Also see - Fuel Induction Service