What Are Coils?
Coils help create a build-up in voltage. Coils in a vehicle can increase voltage from 12 volts to over 25,000 volts. The purpose for this build-up of voltage is to help make the electricity jump from one place to another, like in spark plugs. The spark in spark plugs happens between the spark plug's ceramic/wire to the ground electrode on the metal part of the plug.
Certain high-performance coils can deliver over 40,000 volts to spark plugs of a high-compression modified engine. These high-performance coils take 12 volts and convert it to over 40,000 volts by having more field windings before the current is sent to the spark plugs. As a result these high-performance coils cost more. They also generate a lot more heat in the conversion process so they have oil inside to cool the coil or some other type of cooling process.