What is the difference between polar and non-polar covalent bonding?

Covalent bond = a bond in which the electrons are shared by the partner atoms.

Polar bond = simply a type of covalent bond in which the electrons are shared but not equally. This happens when two different atoms come together, such a C and O. Each atom has a different ability to draw electrons to itself when it shares electrons (that drawing ability is
called Electronegativity).

Polar covalent bond = same as a "polar bond".

Nonpolar bond = two of the same atoms come together, such as the diatomic molecule N2, or triatomics such as O3, etc. A nonpolar covalent bond could be viewed as having "pure" covalent character. There is perfectly equal sharing.

Polar molecule = a molecule in which the polar bonds are disposed in
such a way that it imparts an asymmetry to the molecule as a whole. That
is, it gives the whole molecule a Dipole Moment:

example: water H-O-H has two polar bonds. Since this molecule is
Bent, those two polar bonds ADD TOGETHER to produce a molecule with a
dipole moment. That is, a polar molecule.

example: carbon dioxide O=C=O has two polar bonds also. Since this
molecule is Linear, those two polar bonds ADD TOGETHER to cancel each
other out, to produce a molecule with NO dipole moment. That is, a
nonpolar molecule.

example: carbon tetrachloride CCl4 has four very polar bonds
but these bonds are pointing to the corners of a tetrahedron, and form a very
symmetric arrangement. The polarity cancels itself out, and the molecule is
NONPOLAR.

Contained in chromosomes which are located in the nucleus of the cell.