Voltage is the measure of potential difference in an electrical circuit. Voltage in proper terms can be expressed as the EMF (Electromotive Force) of a circuit. One Volt is the potential needed to cause one coulomb to produce one Jule of Work. Electrons want to be spaced as evenly apart as possible (opposites attract and likes repel). When there are a bunch of electrons jammed in one spot, and a lack of electrons in another spot, the extra electrons will try to go to the "empty" spot. The force that makes them do this is known as Voltage and is measured in Volts. Voltage in a circuit is sometimes compared to water pressure in a pipe. It is the force that makes the electric current flow. Voltage is always measured across a component (e.g. lamp) in a circuit, as it is a difference of potential between two points.