The derived unit of electromotive force, the volt, is the electric potential difference between two distinct points on a wire capable of carrying a charge. More specifically, a potential difference transmits one unit (joule) of energy to each charge (coulomb) that passes through it. More plainly, the voltage can be thought of as the amount of "electric pressure" pushing on electrons.
When dealing with a computer power supply, the voltage a computer uses can vary. For example, in the United States and Canada, a computer uses 120 volts. In other locations like the United Kingdom and Australia, a higher voltage of 230 or 240 volts is used.