The ignition system controls the firing time of spark plugs.  It consists of battery, ignition switch, ignition coil, distributor, spark plug wires and spark plugs.  After the key is released from the START to ON position, electric current flows from the battery to the ignition coil.  The ignition coil converts the low battery voltage (12V) to the high voltage (20,000 to 40,000).  The high voltage current then flows to the distributor.  The distributor delivers the high voltage current through the ignition wires to each spark plug in turn at the exact firing time.  The high voltage current produces a spark across the gap of the spark plug and ignites the compressed fuel mixture in the engine cylinder.