The electrical conduction property of element can be expressed in the terms of energy gap between conduction band and valence band.
Insulator are those material in which valance electron are very tightly bond to there parent atom. Thus, require very large electric field to remove them.
Electrical conductor are those which have overlapping valence and conduction band. It means free electron are available for electric conduction.
A semiconductor is one whose electrical properties lies in between of insulators and semiconductors.