Dielectrics and Polarisation
Dielectrics are non-conducting substances, they have no charge carriers or no free electrons. If an external field is applied, it turns out that charges are induced on the surface which in turn produces a field and opposes the external field. The opposing field does not exactly cancel the external field but only reduces it. Dielectrics are insulators, plain and simple. The two words refer to the same class of materials, but are of different origin and are used preferentially in different contexts.
Dielectrics in capacitors serve three purposes
To understand this concept, one has to look into the charge distribution of a dielectric in its molecular level. The molecules of a dielectric are classified as polar or non-polar.
The "Polar molecules" is one in which the centre of gravity of protons (positive charges) and electrons (negative charges) do not coincide. They have an asymmetrical distribution of charge and have permanent dipole moments in the range 10-30 cm.
Examples : H2O, CO2, NO2 etc.