Voltage is a measure of a electrical potential.
V = W/Q where:
A battery increases the energy of every 10C of charge that is transferred between its terminals, by 15 Joules. What is the battery voltage?
V = ?
W = 15J.
Q = 10C.
V = W/Q, = 15/10, =1.5V.
A potential difference is simply a difference in electrical potential, between two points in a circuit. For example, a potential difference exists across the terminals of a battery. The electrons at the negative terminal, are at a higher potential than the electrons at the positive terminal. (Just as the gravitational potential of a person at the top of a lift shaft, is higher than someone at the bottom).
In circuit calculations ,we use conventional current flow and therefore we regard the positive terminal as having a higher electrical potential than the negative terminal and the charge flowing from positive to negative, (even though we now know this is not really the case).