If it is a computer power supply it is the box that gives the computer its power from the outlet.

A computer power supply has capacitors and can maintain power for a along time.

See:http://en.wikipedia.org/wiki/Capacitor#Hazards_and_safety

The "power supply" supplies power to a computer. A computer's power supply is a "switchmode power supply" responsible for converting the AC voltage from the wall into several DC output voltages.

When your computer will not power on. When the cooling fan of the power supply has failed. When the amount of hardware in use in your case exceeds the power capability of your power supply.

The power supply makes the computer run, without any type of power supply it would be impossible to run a computer.

The mains power supply for a laptop and the power supply unit of a desk top, both convert the high AC voltage from the mains power and convert it to low voltage DC, for the electronics of a computer to use.