What is the difference between GFCI (ground fault circuit interrupter) and AFC (arc fault circuit interrupter)?

GFCI receptacles and breakers turn themselves off in the event of a ground fault. Meaning that if a path to ground exists other than the designed grounding system,(i.e. a person submerged in water, or holding a copper pipe) the circuit will turn off. An AFCI receptacle or breaker turns itself off in the event of an arc. Like if a person were to drive a nail into a wall damaging the wire inside of that wall. This type of protection prevents fires and is required by code in certain applications.