Electrical insulator is a material in which internal electric charges does not freely flow inside it, and therefore make it very difficult to conduct electric current in the presence of an electric field. Agreeing to the electronic band theory “a charge electron flows if the states are available into which electrons can be excited and jump which allows them to gain energy and thereby move through spaces inside a conductor such as a metal. If no vacant states are available and no such material is available, then material is an insulator.”
Mostly insulators have a large energy band gap. This happens because the “valence” band which have highest energy electrons is completely full, and a large energy gap divides this band from the next band which lie above it. There is always some presence of voltage which is called the breakdown voltage that gives electrons enough energy to be excited and jump into this band. Once this voltage is surpassed the material ceases being an insulator, and charge begins to pass through it.
A dielectric is also an electrical insulator that can be polarized with the help of electric field by applying on it. When a dielectric is placed within an electric field, electric charges do not flow or move through the material as they do within a conductor, but only marginally shift their position from their average equilibrium position causing dielectric polarization. Due to dielectric polarization, positive charges are moved toward the field and negative charges shift their position in the opposite direction which creates an internal electric field that reduces the whole field within the dielectric itself.