Voltage is a force that makes electricity move through a wire. It is measured in volts. Voltage is also called electric tension or electromotive force (EMF). It was named after Alessandro Volta.
Technically, the voltage is the difference in electric potential between two points. Voltage is always measured between two points, for example between the positive and negative ends of a battery, or between a wire and ground.
As seen in volt#Hydraulic analogy, voltage can be seen as the pressure on the electrons to move out of the source. It is directly proportional to the pressure exerted on the electrons. In other words, the higher the voltage, the higher the pressure. For example, a battery of 3 volts will exert pressure on the electrons twice as hard as a battery of 1.5 volts.
The voltage can push the electrons into a component, like a resistor, creating a current. Usually, the voltage and the current are related by a formula (see impedance).
Note that there must be both voltage and current to transfer power (energy). For example, a wire can have a high voltage on it, but unless it is connected, nothing will happen. Birds can land on high voltage lines such as 12kV and 16kV without dying because the current does not flow through the bird.
There are two types of voltage, DC voltage, and AC voltage. The DC voltage (direct current voltage) always has the same polarity (positive or negative), such as in a battery. The AC voltage (alternating current voltage) alternates between positive and negative. For example, the voltage from the wall socket changes polarity 60 times per second (in America). The DC is typically used for electronics and the AC for motors.