It is not only concerned with the "electro–optic effect", since it deals with the interaction between the electromagnetic (optical) and the electrical (electronic) states of materials.
This interaction usually results in a change in the birefringence, and not simply the refractive index of the medium.
In a Kerr cell, the change in birefringence is proportional to the square of the electric field, and the material is usually a liquid.
In a Pockels cell, the change in birefringence varies linearly with the electric field, and the material is usually a crystal.
Non-crystalline, solid electro-optical materials have generated interest because of their low cost of production.