The spark plugs are screwed into the cylinder head, which means that the end of the plug is sitting at the top of the cylinder where the action happen. At just the right time (thanks to the distributor), when the intake valves have let the right amount of fuel vapor and air into the cylinder, the spark plug makes a nice, blue, hot spark that ignites the mixture and creates combustion.
At this point, the ignition system has done its job, a job it can do thousands of times per minute.
