The plug has three main Parts, the centre electrode, the ground electrode and the insulation separating them. Besides these, these are the body shell, the sealing ring and the gasket washer. The upper end of the centre electrode is connected to the brake chamber terminal, where H.T. cable from the ignition coil in case of single cylinder engines (or from distributor in case of multicylinder engines) is connected. The lower end of the centre electrode projects beyond the insulator to form a gap with the ground electrode. The insulator is meant to fulfill the functions.

More details would be given if you want to learn more. Next time, I will tell you haw to choose a proper spark plug and how to maintain and check spark plug.