Answer:
A polar covalent bond will have the electrons shifted more towards one side or the other; a nonpolar bond will have evenly distributed electrons.
Explanation:
The basis for a polar bond is that the electrons are shared unevenly. Some elements are more electronegative than others; that is, they pull harder on the electrons and tend to have more electrons around them. Following the periodic trends, Fluorine is the most electronegative element, so it will likely have the most dense area of electrons in a molecule. All this means that in a polar molecule, the electrons will be spread out unevenly along the lengths of the bonds as some elements will pull on them more than others will.
Nonpolar molecules do not have to worry about differences in electronegativity, and so the electron density will be pretty evenly distributed along the entire molecule.
Hope this helps! :)