DriverSide's Car Dictionary has the most comprehensive definitions of automotive terms. This auto glossary can help you decipher what your mechanic is fixing and what the purpose or function of the particular auto part or accessory is for. We have taken the mystery out of automotive service jargon, so that you don't have to.
Along with oil and oil filters, spark plugs are the face of automotive maintenance, devices that produce an electrical spark to the combustion chamber in order to ignite the air/fuel mixture. (Your lawnmower probably has one too, if that interests you.) Replace these per your manufacturer recommendations, though you can replace them sooner if you have a condition in the combustion chamber that keeps ruining the plugs. In fact, in many ways the spark plugs (along with fuel efficiency) can be considered a window into the health of your engine, and can be removed and inspected to have a look-see into what's going on in the combustion chamber.