What are polar and non polar molecules.

The molecules in which the "centre of gravity" of nucleus does not coincide with the centre of gravity of revolving electrons since they have a permanent dipole moment.
eg. $H_O,N_{2}O$ etc are polar molecules
The molecule in which the centre of gravity of molecule coincide with elections are known as non-polar molecules they don't have permanent dipole moment
e.g. $H_2,N_2$ etc.