How are power supplies in computers rated and why?

Power supplies are rated on the power requirements of the motheboard and add-on devices such as hard drives and CD-ROM drives.

Why? Because the motherboard uses a certain amount of power, that depends on the CPU and RAM power requirements, as well as what video card is connected and the NIC that is used. Hard drives and CD-ROM drives are pretty typical for their power requirements, and draw their power directly from the power supply.