We know that a random motion of free electrons results from the exchange of the outer orbit between the atoms. In order to establish a net or average flow of electrons in one direction an external force is necessary. This force is necessary to overcome a resistance. If the material is a good conductor of electric charges, relatively smaller force is required to produce a current flow.Copper and silver are good conductors. Silver is better conductor than copper.
A good insulator is one which effectively eliminates the flow of current of any practical magnitude. Polystyrene is a good insulator and has approximately 3 x 1018 free electrons percubic metre. Materials which have 1021 to 1025 free electrons per cubic metre are known as semiconductors.