Imagine you have two plates of iron; one has a negative charge, and one has a positive charge. What will happen when the two plates are placed into contact?
Electrons will spontaneously flow from the negatively charged plate to the positively charged plate, until finally there isn't a difference in charge between the plates.
You'd have to do some work to reverse the process. The electrons rushing towards the positive plate have some 'push', and you would have to expend energy to keep them on the negative plate. The voltage is a measure of how much push the electrons have.
Voltage is related to the amount of energy released every time a certain number of electrons jump from one of the plates to the other.
If you measure the energy released when 6.2 billion billion electrons jump from one plate to the other,you have measured the voltage between the plates. For example, suppose that 6.2 billion billion electrons release 2.1 joules of energy when they flow between the plates. The voltage between the plates is 2.1 Volts.
A voltage exists between any pair of objects with different charges. If the charges on either side of a boundary aren't placed exactly the same way, you'll get a voltage across the boundary.
Author: Fred Senese firstname.lastname@example.org