The term spark plug coil can refer to a couple of different items. In general, an ignition coil is a component that generates spark in an internal combustion engine. This component is sometimes referred to as a spark plug coil because it is where the high voltage electricity required to jump the electrode gap of the spark plugs is generated. The term can also refer to a coil-on-plug (COP) unit, which is a specialized type of coil pack that is integrated right into the spark plug boot. It can also refer to other distributorless ignition system (DIS) coil packs, depending on the application.
In a traditional internal combustion engine, the fuel is ignited when high voltage electricity crosses between the electrodes of a spark plug. This electricity can be generated in a couple different ways, though one common method uses a coil that contains two sets of windings. Low voltage electricity passing through one coil is used to induce a high voltage in the other, which in turn travels through the spark plug, across the gap, and grounds out in the cylinder head.
Many older vehicles use a single coil to fire each spark plug. This coil is connected to a distributor cap by a coil wire, and the distributor rotation determines which cylinder the electricity goes to. A rotor inside the distributor cap spins around with the engine, and if it is properly timed it will complete a circuit to a spark plug wire just as the corresponding cylinder reaches top dead center (TDC).
Newer vehicles often use distributorless ignition systems and can have one coil per cylinder. Others use a waste spark system, where a single coil fires two cylinders at once, one on the compression stroke and the other on exhaust. When each cylinder has its own, the term spark plug coil is sometimes used. These coils can be contained together in a physical pack, though they can also be in any other configuration or there can be just one coil.
One particular type of spark plug coil locates each coil pack directly on a plug boot. This coil-on-plug system does away with spark plug wires altogether because the spark is generated right at the boot. There are still wires connected to the boot, but they only carry a low voltage for the spark plug coil to transform into the necessary high voltage spark. Unlike many spark plug wires that simply clip on to the spark plug, these combination units are often bolted down to the valve cover.