What causes electric shock?

Electric shock is caused by a difference in conduction of electricity across different surfaces. Electricity will always find the path to ground (or the completion of a circuit) that offers the least amount of resistance. When any object providing less resistance than the normal circuit enters the path of the electrons, the current will leave the circuit and travel through the new path to ground. This principle is why birds can stand on electrical wires and be safe, yet a downed electrical wire can be deadly for several hundred feet in all directions. Electricity dissipates in various and unpredictable ways. If you have a difference in voltage from one place to another, and there is a circuit completion (a wire, a finger) in between, current will jump across. Electric shock is not the state of being electrically energized, that can be totally harmless if controlled; it is the state of completing a circuit, or bridging the gap between conductors.