In practice anything that interrupts or affects the continuous flow of normal power can cause a surge or transient overvoltage to be induced onto the mains supply to your electrical equipment.

Lightning discharges an enormous current flow that in turn creates an electromagnetic field. If power or data cabling passes through the electromagnetic field, the cable will pick up a voltage or alternatively an induced voltage will affect the cable.

This occurs when lightning discharges close to overhead power or telephone lines.

For the really technical answers please refer to the American National Standard IEEE C62.41-1991 and British Standards BS6651: 1992.