The spark plug is the final step for the ignition process. All of the voltage and energy build up to make a final leap across the plug gap which initiates combustion of the fuel mixture.
The spark travels through the plug wire terminal to the threaded tip of the plug’s center terminal. This terminal reaches all the way through the ceramic shell of the plug and ends at the electrode. The electrode is centered in the plug with a gap between it and the tip of which is connected to the metal housing of the plug forming a ground to the engine. The voltage jumps this gap to reach ground, resulting in a spark that ignites the air/fuel mixture in the cylinder.
One physical adjustment you can make with most plugs is the distance of the spark plug gap. Opening the gap forces the coil to build up a higher voltage in order for the spark to jump the gap. Too large of a gap will also put added pressure on the secondary side of the ignition system, resulting in shortening the life of the components. Too small of a gap can make life too easy on the ignition to the point where the resulting in a weak spark that may not be strong enough to create a full combustion cycle of the air/fuel mixture.
There are a variety of spark plugs available from simple to exotic designs. And of course each one has their own theories on why one is better than the other. There are platinum-tipped plugs, plugs with tiny electrodes, some with numerous electrodes, and more. There are also spark plugs called resistor plugs, non-resistor plugs, hot plugs, and cold plugs. It is best to follow your engine builder’s recommendation for plugs, or to test and tune with gaps and different brands.