The power supply unit (PSU) is essentially a fancy transformer that accepts a range of input alternating current (AC) voltages (usually from 90VAC to 240VAC), and provide a number of regulated direct current (DC) voltages (-12V, -5V, 3.3V, 5V and 12V).
Each PSU has a power rating. The power rating of a PSU states how much power the PSU can supply on a sustained basis. Unfortunately, many manufacturers falsely report the power rating of PSUs. This means that some PSUs cannot sustain a power consumption at the rated number.
Note that although a PSU is not glamorous, it significantly affects the reliability of a personal computers. A poorly engineered PSU can cause a PC to be flaky at the least. Depending on the mode of PSU failure, it can provide voltages that are high enough to destroy all components in a personal computer!