Conductors: The substances which allow heat to pass through them easily are called the conductors of heat.
For example: Silver, copper, iron, aluminum and mercury are conductors of heat.
The substance like glass, stone, marble and slate do not conduct heat easily as metal do. These substances are not as good conductors of heat as the metals are.
Insulators: The substances which do not allow heat to pass through them are called the Insulators (or bad conductors) of heat.
For example: wood, plastic, mud, cork, cotton, and wool, asbestos, still air are insulators of heat. All liquids (except mercury) and gases are bad conductors of heat