Every substance, even air, will conduct an electrical current if enough voltage is applied to it, but the word 'conductor' normally is used for materials that allow current flow with little resistance.
Most metals are good conductors. The most common conductor is copper. It's used in virtually all the wiring that connects automotive components together. The heavier the current a conductor has to carry, the heavier the gauge or thickness of the wire.
Materials that don't conduct current easily are insulators. Most plastics are good insulators. The plastic covering on a wire is an example of this. The ceramic portion of a spark plug is also a good insulator.
