The measurement of the diameter of wire. As the gauge number increases the size of the wire decreases. As the wire gauge number decreases, the diameter of the wire increases. For example, an 8 gauge wire is going to be larger than a 16 gauge wire.
When purchasing wire, make sure that you know what size, or gauge of wire is needed.
The material that the wire is made of. Copper & aluminum are the most commonly used materials. OFC - Oxygen Free Copper is the ideal choice for wire material. Copper is a superior electrical conductor, & does not expand or contract with heat. CCA - Copper-Clad Aluminum is a suitable choice for lower powered or budget applications. The power transfer is not on par with OFC, but this material offers significant savings.
Since 1857, The AWG (American Wire Gauge) system has been the standard for measuring the diameter of wire. AWG standard is the measure of bare wire to find true gauge. Companies that do not follow AWG Standards include the thickness of the wire jacket which means the actual wire is not "True to Gauge".