A spark plug is a device used in an internal combustion engine — that is, an engine that derives its power via exploding gases inside a combustion chamber — to ignite the air-fuel mixture.
Cars typically have four-stroke gasoline engines, which means there are four strokes, or movements, to the moving parts inside the engine per rotation. Inside each cylinder is a piston, which moves up and down within the cylinder to compress the gas for combustion, and pushes the exhaust gases out after combustion. The piston is operated by an arm that attaches to the crankshaft, a shaft that extends through the bottom of the engine. The piston goes up and down twice during each rotation, hence the four strokes -- up, down, up, down.
The spark plug is positioned at the top of the cylinder, where the air-fuel ratio is compressed. The tip of the plug sits inside the engine, recessed into the side of the cylinder wall. The other end remains outside the engine and is attached to a wire that is also attached to the distributor.
As the piston travels up the first time, compressing the air-fuel mixture inside the combustion chamber, the distributor sends an energy surge along the spark plug wire. When the surge reaches the plug, it fires, igniting the air-fuel mixture. The explosion inside the combustion chamber forces the piston down again, turning the engine further as the same process happens in all of the other cylinders, one at a time. As the piston comes up the second time, the exhaust valve opens, and the burned gases are released into the exhaust pipes.
As you can see, the spark plug is a very important part of the engine. Therefore, in order to maintain the highest performance a car can offer, care should be taken to perform the proper scheduled maintenance in a timely fashion. Most older cars have copper spark plugs, which require yearly maintenance, usually performed at each tune up.
The best course of action is to replace each spark plug during a tune up. However, copper plugs can also be cleaned with emery paper, which is a very fine sandpaper that polishes soil from the plug's tip. Whether replacing or cleaning the plugs, each spark plug should be gapped to the correct specifications, usually found in the owner's manual or shop manual; it will not operate correctly if there is too much or too little space between the tip of the plug and the metal finger that curves over the top of it.
On the other hand, a platinum spark plug requires much less maintenance. Some platinum plugs can last 100,000 miles (160,934 km); if there is a discrepancy between the recommendations in the owner's manual and the manufacturer, adhere to the more conservative mileage rating. Like copper spark plugs, platinum versions need to be gapped to the car manufacturer's specifications. However, emery paper should never be used on a platinum spark plug; if the plug becomes soiled, it will simply need to be replaced. For this reason, regardless of the maintenance timetable, each plug should be removed and inspected at every tune up.