The power supply converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer. In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan. Typical computer power supply generates the voltages needed by the computer motherboard accessories. A typical modern PC power supply generates the following voltages:

* +5V (+-5%) at up to tens of amperes for motherboard electronics, disk drives and cards
* +12V (+-10%) at several ampreres for disk drives and some cards
* +3.3V (+-5%) up to tens of amperes for the majority of modern logic electronics in motherboard
* -12V (+-10%) usully less than one ampere for some accessory cards

Most other computer power supplies usually give voltage on this line, because those are the most commonly used voltage used in computer systems. Depending on the PC model, power supplies are rated anywhere between 150 and 350 W. A PC power supply on average is rated for 250-400 watts. PSUs above 300W are unusual and only tend to come in servers, or machines that have been designed for ‘hardcore’ applications such as gaming, where a hundred watt graphics card is not that unusual.
Source: http://powersupplycircuit.blogspot.com