If watts is power and amps is current, what is volts
Voltage is the difference in energy between two points in an electric field, expressed per unit of charge. A Volt is a Joule per Coulomb: \$V = J/C\$.  The voltage between two points tells us how much energy each electron will gain or lose when it moves between those two points. 

The separation of opposite charges stores energy. If we separate charges such that there is one volt between them, that represents less energy than if we separate the same charges such that there are two volts between them.  And of course we store more energy by separating more charges at the same voltage.

This is why power is related to both current and voltage.