The spark plug quite simply forces electricity to arc across a set gap to ignite the contents of the combustion chamber at the required time.
Plugs are required to run at very high voltage to create good spark, The voltage is amplified by the ignition coil upon sparking.
The picture below highlights the key components of a plug.
Terminal - This is what the plug is powered from, The HT lead takes power from the coil to the plug via this point.
Ceramic insulator - Spark plugs have a ceramic insert to isolate high voltage at the electrode, making sure that the spark happens at the tip of the electrode and not on any other part of the plug.
Hex head - The hex head is used for installation and removal of the plug.
Plug seat - This seat is basically a gasket between the plug and engine block. The washer is slightly crushed upon tightening down creating a very good seal.
Reach - Exactly as it states, this quite simply is how far the plug 'reaches' when fitted.
Ground electrode - The negative electrode that earths via the body of the plug to the motor itself
Centre electrode - The positive electrode carries the power usually via a copper core in the plug from the terminal
Gap - This is the distance between the centre and ground electrode (these days set by the manufacturer).
Thread diameter - exactly what it states, used to ensure correct application.
The following sparkplug specifications apply: