The alternating current that powers ordinary household appliances changes polarity 60 times per second, so it's meaningless to speak of positive and negative wires. Instead, the "hot" wire leads from the power source while the "neutral" one provides a return path. There are several conventions for distinguishing these wires on an appliance cord.
Prongs on Polarized Cords
Appliances require polarized cords when it makes a difference which terminal inside the appliance is hot and which is neutral. Most light fixtures, for example, require polarized cords, because only the base of the socket should be hot. If polarity is reversed, and the socket lining becomes hot, you could get a shock by touching the socket. Plugs for polarized appliances have a wide prong -- which is neutral -- and a narrow prong, which is hot. If the plug has a third prong, it connects the appliance to a ground circuit.
Identifying Hot and Neutral Wires
Many appliance cords consist of stranded copper encased in plastic insulation. If the cord is polarized, and the colors of the wires are the same, the neutral wire either has ribbing or a white stripe to identify it. If the wires are color-coded, the hot one is either black or red; the neutral wire is always white. If there is a ground wire, it is either green or bare.