A three-prong wire outlet has parallel slots with a hot and a neutral wire, plus a ground slot below the others to prevent electrical shocks from appliances with metal casings. It accepts both two- and three-prong plugs.
Electricity flows from the outlet's hot wire to power the appliance, then back through the outlet's slightly larger neutral slot and ultimately to the ground. A power surge from a malfunction trips the circuit breaker and cuts off the flow of electricity.
Damage to the wires going into and out of a motor causes electricity to flow through any metal the wires touch, shocking anyone who comes in contact with that metal. A third wire in the power cord of appliances with this risk runs from the metal housing to the outlet's ground slot and diverts electricity to trip the breaker and prevent electrocution.