Power supply unit

The power supply unit does what it says on the tin, it supply's power to the components in the computer. It does this by getting power from the mains that is plugged into it at the back, it then converts this AC to low voltage DC power that the internals of the computer can use. There are cables coming out from the PSU that are then plugged into components to power them. Larger wattage of power supply units allows for more components in a computer to be powered, higher end components might require more power than your PSU can handle.