What are polar & nonpolar dielectric

(a)Polar Dielectric : Those substances in which the centre of positive and negative ions of molecules are not concentrated but are separated, are called polar dielectric substances. Each molecule of these substances have net permanent electric dipole moment and each molecule behaves like an electric dipole. For example, HCl molecule  

In absence of an external electric field, the molecules of a polar dielectric substance are arranged in irregular manner due to thermal energy. Due to this, any volume of a substance which has molecules in large amount has zero resultant dipole moment and thus, the resultant dipole moment of the substance is zero. On placing the substance in external electric field, each molecule applies torque on the dipole that tries to align the molecule along the direction of external electric field. On increasing the electric field intensity of the external electric field, more of the molecules get align themselves in the direction of the external electric field. Thus, there is net dipole moment in the substance.

(b)Non−polar Dielectrics : Those substances in which the centre of both passive and negative charges of the molecules are coincide, are called non-polar dielectric substances. For example, H2​, CO2​, N2​ and O2​ etc.  Each molecule of these substances has zero dipole moment. Thus, the net dipole moment of the whole substance is zero.

If a non-polar dielectric substance is kept in an external electric field, then the centre of the positive charge is along the electric field and the centre of the negative charge is displaced opposite to the direction of the electric field. this induces some dipole moment in each molecule.
"The induction of electric dipole moment in a dielectric substance in the presence of external electric field is called the polarization of dielectric substance.