A spark plug is an electrical device used in an internal combustion engine to produce a spark and ignite the fuel-air mixture in the combustion chamber. As part of the engine's ignition system, the spark plug receives high-voltage electricity (generated by an ignition coil in modern engines and transmitted via a spark plug wire) which it uses to generate a spark in the small gap between the positive and negative electrodes. The timing of the spark is a key factor in the engine's behavior, and the spark plug usually operates shortly before the combustion stroke commences. The spark plug was invented in 1860, however, its use only became widespread after the invention of the ignition magneto in 1902. Diesel engines use compression ignition (instead of spark ignition), therefore they do not normally use spark plugs.

Source: Wikipedia