Voltage is the electrical force that causes free electrons to move from one atom to another. Just as water needs some pressure to force it through a pipe, electrical current needs some force to make it flow. "Volts" is the measure of "electrical pressure" that causes current flow. Voltage is sometimes referred to as the measure of a potential difference between two points along a conductor.
Voltage is typically supplied by either a generator or battery. Generators are analogous to a water pump in a water piping system, and batteries are similar to water towers. Both systems have a potential difference between the source of the power and someplace downstream from the source.
The scientific symbol for voltage is an "E", dating to early days of electricity when it was called the "Electromotive force". Scientists and engineers use the "E" symbol for voltage, while electricians and wiring books use "V" as the voltage symbol. This can create some confusion, since either may be encountered. In this title, we'll use the practical symbol "V" for voltage.