American wire gauge  basically tells you how thick a cable is. Most wires will have the wire gauge printed on them. The lower the number, the thicker the cable. A thicker cable has less resistance, and thus will have a lower voltage-drop.
Factors in voltage drop are Length and Amperages, i.e how much power the PCB draws.
Used for control-panel wiring.
Used on some Jamma Harnesses for both power and controls, and the thinner cables on an ATX psu.
Used to power the Naomi PCB, and ATX psu's.