Power is the rate of "doing work". It can also be described as the rate of energy transfer, which is the same thing.
Power = Energy Transferred / Time
Power is therefore a measure of how much energy is being transferred into another form per second. It is measured in Watts (W). It is sometimes useful to rearrange this equation to read:
Energy Transferred = Power x Time

In electrical terms, and this applies to any transfer of energy which goes through electrical energy (generators, motors, etc.), the power can also be calculated using the equation:
Power = Voltage x Current
The amount of energy transferred is therefore (putting the two formulas together):
Energy = Voltage x Current x Time

To address the other possible answers, voltage x current x time = energy, not power. It is not just when electrical enegry is transferred to another form of energy, it can also be the other way around. Joules is a unit of energy, not power.