The power supply is the part of the computer the runs your computer system. There are several cables that run from the power supply. One cable will attach to the main board, one will attach to the CPU, and HD, another cable will attach to DVD, CD ROM. You want to make sure you have the right size of power supply if you are building your computer. After all the power supply is the best preforming part, so you can get the most out of your computer.