The thickness of the wire found in a power cord is what AWG (American Wire Gauge) pertains to. 18 AWG and 14 AWG are typical sizes for computer power cords. In other words, the size of the wire determines how much current can flow through that cable at any given moment; smaller numbers mean thicker wires with larger current capacities while bigger numbers stand for thinner wires with lower current capacities. Mostly, 18AWGs will do for desktop computers and their peripherals but if you have more demanding needs such as those involving higher amperage ratings then it might be necessary to use 14AWGs instead.