Answer:
Potential difference is the work done in moving a positive test charge from infinity to the point in question.
Voltage is an expression of PD. (Joules / Coulomb)
Say that a capacitor has a PD of 5 Volts. The work in moving a positive test charge from the positive plate to the negative plate is -5 Joules/Coulomb or -5 volt. (At the positive plate the positive test charge (1 Coulomb) already has a PD of + 5 Volts.)
