Drift

The procedure in which charged particles move under the effects of electric field.

In the given figure there are few electrons exist nearby valance band if we apply some electric field to such type of material then these electrons because of applied electric field can drift therefore we may  observe some action into electrical property of the material so some flow of current also monitor and we say this a conductor.

In the given diagram all the energy levels are completely filled with electron if we apply any electric field to such type of material then these electrons remain no space to drift therefore such material behave as insulator. Insulators are such type of material which offers highest resistance to the flow of electrons and their conductivity is very much low.                                              

In the above diagram almost all of energy levels are full except two levels are kept vacant if we apply electric field to such type of material then these electrons contain some space to drift hence we will monitor some flow of current  through such type of material so these material are known as conductors. Conductors are such type of materials s which offers zero resistance to the flow of electrons and their conductivity is highest.