Conductor -band structure

In a conductor the valence bands are not full and in effect the conduction band is continuous with the valence band. Thermal excitation ensures that the conduction band is always populated to some extent. 

Under the influence of an applied electric field, an electric current will flow. Electrons in the valence band will move to the conduction band, leaving behind a positive charged vacancy called a hole. This hole will be immediately filled by another electron, leaving another hole. The holes migrate in the opposite direction to the electrons. The movement of the electrons and holes in opposite directions contributes to the observed conductivity of the material.

See insulator and semiconductor for comparison.