The power supply supplies the electrical power for a computer. It supplies power to the motherboard, drives, and certain expansion cards. It normally has at least one fan that helps cool the power supply and will assist in the task of cooling the computer. Some power supplies have an additional outlet on the back that can be used to provide power to the monitor. Power supplies come in a variety of wattages. They range anywhere from around 160 watts to about 700 watts. 350 to 400 watt power supplies are probably the most common. A higher wattage power supply doesn't hurt anything, but a lower wattage power supply can cause problems for people with lots of devices connected to their computer.