Bond Polarity

For molecules to be polar, it means that there is a difference in electronegativity and that their dipole moments don't cancel each other out. This is caused by one atom exerting more of a force on the electron cloud than the other. An electric dipole is the unequal sharing of electrons within a bond. Partial charges are marked by delta plus and delta minus. This is shown below:

Source

If the only bond in a molecule is polar, the molecule is polar. If the bond is nonpolar, the molecule is nonpolar. If it is nonpolar, there is no difference in electronegativities, the dipole moments cancel, and they have a symmetric shape. In molecules with more than one bond, both the shape and bond polarity determines if it's polar or not. 

Here are a couple links to practice problems for what is explained above:

-Practice 1

-Practice 2