HardwareSecrets have posted a tutorial on the elusive PSU. Everything you ever needed to know about them, including types, how to calculate its true power, efficiency, power factor correction (PFC) and more.

" In other words, the power supply converts the conventional 110V or 220V alternating voltage into continuous voltage used by the PC electronic components, which are: +3,3V, +5V, +12V, -5V and -12V."