Conductor, Semi-Conductor And Insulator

 Conductor

Any Material which allows to flow the electric current through it is called as Conductor
Example: Copper,Aluminium

Semiconductor

The material which allows to flow the electric current in some condition
Example: Silicon,Germanium

Insulator

A material whose internal electrical charges do not flow freely and therefore make it nearly impossible to conduct an electric current under the influence of an electric field
Example: Rubber,Plastic,Glass

Atoms with less than four valence electrons are good conductors 

Atoms with more than four valence electrons are poor conductors 

Atoms with four valence electrons are semiconductors