The ignition distributor has several functions - its first function is to distribute high voltage from the coil to the appropriate cylinder. This is done via the ignition distributor cap and rotor. The ignition coil is connected to the distributor rotor, which spins inside the distiutor cap. The rotor spins past a series of contacts, one contact per cylinder. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder. When you do a tune-up, one of the things you replace on your engine is the cap and rotor -- these eventually wear out because of the arcing. Also, the spark-plug wires eventually wear out and lose some of their electrical insulation.