Unequal sharing of bond pair of electrons between two atoms in a molecule forms 

Bond which forms between the atoms through sharing of valence electrons are known as covalent bond.
There are two types of covalent bond:
(1) Polar covalent bond: When there is unequal sharing of electrons between the bonded atoms.
(1) Non-polar covalent bond: When there is equal sharing of electrons between the bonded atoms.