A molecule is polar when the average of the charges and the masses do not match.

When the bonded atoms have different electronegativities (different ability to attract the bonding electrons) the electronic binding pairs move towards the more electronegative atoms, thereby generating a dipole (separation of charges).

Depending on the geometry of the molecule and the arrangement of the bonds, the dipoles may be canceled resulting in a non-polar molecule even if the links are polar.

In conclusion, a molecule will be polar if contains polar bonds (between atoms of different electronegativity) and bond dipoles do not cancel.