Voltage (also called electrical potential difference or electric tension or electromotive force) is the potential energy that makes the electrical current flow in a circuit by pushing the electrons around. Voltage is equal to the work which would have to be done, per unit charge, against a static electric field to move the charge between two points.
Voltage can be direct or alternating. A direct voltage maintains the same polarity at all times. In an alternating voltage, the polarity reverses direction periodically. The number of complete cycles per second is the frequency, which is measured in hertz (one cycle per second), kilohertz, megahertz, gigahertz, or terahertz. An example of direct voltage is the potential difference between the terminals of an battery. Alternating voltage exists between the terminals of a common utility outlet.
The unit of voltage is volt shown as 'v'. One Volt is equal to one Joule of energy that can move one Coulomb of electrical charge. A voltmeter can be used to measure the voltage (or potential difference) between two points in a circuit.