Insulator: A material with a substantial gap between the valence and conduction bands. Because no electrons can travel up to the conduction band, the valence band is filled. As a result, the conduction band is completely vacant.
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Conductor: The valence bands of a conductor are not complete, hence the conduction band is continuous with the valence band. Electrons in the valence band will migrate to the conduction band, leaving a positive charged vacancy known as a hole behind.
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Insulator Defintion:
Energy band Theory:
According to this theory the overlapping of valence band and conduction band tells us about the conductivity of a substance, in insulators the valence band and the conduction band both are separated to a large distance which will cannot be over lapped by applying small energy. If conduction band and valence band don't overlap there would be no exchange of electrons and in that case the material cannot conduct electricity or heat.
Presence of Free Electrons:
The conductivity of a element is wholly and solely dependent upon the presence of free electrons which which can move freely and conduct electricity . In insulators there are no free electrons and in that case it cannot conduct electricity.
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Conductor Definition:
A conductor is an object or kind of material in physics and electrical engineering that permits charge (electric current) to travel in one or more directions. Metal materials are common electrical conductors. The materials or substances that allow electricity to flow through them are referred to as conductors. Furthermore, conductors allow heat to pass through them. Metals, the human body, the Earth, and animals are all examples of conductors. The human body is an excellent conductor.