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what is polar molecules and non polar molecules and momentary dipole?

A polar molecule has a difference in electro negativity. So that there is a slight +ve charge and -ve charge.
Example : water

A non-polar molecule shares electrons equally and does not have oppositely charged ends.
Example : CO₂, H₂, N₂, Cl₂, O₂, etc.

Momentary dipole : Momentary dipole is created when electron shared by atoms are distributed unevenly (because of big difference in electro negativity of the atoms) in the molecule or functional group, creating fractional charges.

For example: Momentary dipoles are created between hydrogen and oxygen atoms as the electro negativity between those atoms is quite big. Therefore electrons spend more time closer to more electronegative oxygen, than hydrogen, creating a fractional negative charge on the oxygen side and fractional positive charge in the hydrogen side.