A conductor is an electrically conductive transmission medium on which signals are transmitted by electron movement. Conductors are all materials that transport current with free electrons. Depending on the material, the electron movement occurs with less or more resistance. Materials with relatively free electron movement have high conductivity, including silver and copper, while others with stronger electron bonding are poor conductors, such as coal.
The decisive factor is the band gap, which is the energetic distance between the conduction band and the valence band. In conductors, the two bands are immediately adjacent. It is different with non-conductors. These materials have no free electrons for current transport, so they are not conductive, and the band gap is very large. There are also semiconductors, which are located between conductors and nonconductors, and superconductors, whose resistivity jumps to zero at cryogenic temperatures.