Voltage is the electrical force or "pressure" that causes current to flow in a circuit. It is measured in VOLTS (V or E).
In other words, current can only flow only if voltage is applied between two terminals. The difference between these two terminals is called the potential difference. The larger the potential difference, the larger the voltage.
Therefore, Voltage can be thought of as the measure of the pressure pushing the electrons.
One volt will cause 1 amp of current through 1 ohm of resistance. 
Note: Resistance will be discussed in a later section
