An electrical conductor is a medium through which an electric current will flow. Metals are generally very good electrical conductors. The best of all is silver but, for reasons of economy, copper is most often used. Most nonmetals are poor conductors; however, there are exceptions such as plasma. Non-conducting materials are known as electrical insulators.
The electrons in conductors lie in a so-called conduction band that, in an energy diagram, sits on top of the valence band. Electrons in the conduction band are able to move through the medium when a potential difference is applied between two points.
Under normal conditions, all materials offer some resistance to flowing charges, which generates heat. In most cases, electrical resistance rises with increasing temperature.
A material can be an electrical conductor without being a thermal conductor, although most metals are both.