What is the difference between a nonpolar covalent bond and a polar covalent bond?

Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. For example, water is thought to be polar because of the unequal sharing of its electrons. However, methane is considered non-polar because the carbon shares the hydrogen atoms uniformly.

Nonpolar covalent bonds form when electrons are shared equally. When the two atoms sharing electrons are identical, such as in oxygen gas (O2), the electronegativities are identical, and both atoms pull equally on the electrons.
Polar Covalent bonds form when electrons are shared unequally. Atoms in this kind of bond have electronegativities that are different, and an unequal distribution of the electrons results. The electrons forming the bond are closer to the atom with the greater electronegativity and produce a negative charge, or pole, near that atom. The area around the atom with the weaker pull on the electrons produces a positive pole. In a molecule of water(H2O), for example, electrons are shared between the oxygen atom and each hydrogen atom. Oxygen, with a greater electronnegativity, exerts a stronger pull on the shared electrons than does each hydrogen atom. This unequal distribution of electrons crates a negative pole near the oxygen atom and positive poles near each hydrogen atom.

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