When electrons are not shared equally in a covalent bond, the bond is called a polar covalent bond. It is possible for a molecule as a whole to be nonpolar even if it has multiple polar covalent bonds.
A covalent bond occurs between nonmetal atoms that seek to share outer electrons in order to complete their shells and become more stable. All atoms have a certain affinity to electrons, measured by their electronegativities. If one atom has a much stronger electronegativity than the other, it will exert a greater pull on the shared electron. This results in a slightly negative charge on one atom and a slightly positive charge on the other, known as a polar bond.