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Spark Plugs

Spark plugs are very important in gasoline-burning engines: their spark is what ignites the fuel-air mixture and provide combustion¹. Engine timing strives to have the fuel-air mixture at the correct compression when the spark is delivered from the plugs²: if the fuel-air mixture begins combustion too early before the spark is delivered from the plugs, the engine suffers a "detonation" (recognisable by an audible ping); if the fual-air mixture does not begin combustion, the engine suffers a "misfire".

Although there are various causes for engine detonation (fuel octane rating too low for the engine design) and engine misfiring (obstructed fuel injectors), the most common and most preventable cause is the engine spark plugs themselves: spark plugs crack, spark plugs arc and spark plugs get dirty. Changing spark plugs and their cables is simple and inexpensive and can often solve engine problems. However, not all spark plugs are created equal and neither are their cables.

Distributor

Something which is often overlooked when diagnosing problems related to the spark plugs is the distributor: a mechanical device directing the spark plugs when to spark³. There are 2 moving parts on a distributor which wear and tear and need regular replacing: the rotor and the cap. The distributor spins the rotor and sends electricity through it. At its tip, the distributor rotor makes contact with the distributor cap, where some cables run to the spark plugs. When the distributor rotor tip touches the end of a cable through the distributor cap, it sends electricity through that cable, making plugs spark. When the distributor rotor tip and/or the distributor cap electrical contacts for the distributor rotor tip are worn down, the electricity cannot be conducted as efficiently and some of the spark plugs may "skip their turn" on occasion, and you have an engine misfire.

References

1. Wikipedia > Spark Plugs
2. How Stuff Works > Ignition System > Timing
3. How Stuff Works > Ignition System > Distributor