Polar and non-polar molecules
A substance that contains polar covalent bonds may not be overall polar. This is due to the shape of the molecule.
Water molecules are polar molecules. Both of the bonds inside the molecule are polar bonds.
Due to the non-symmetrical shape of the molecule (bent), the molecule itself is polar (it has a \(\delta-\) and \(\delta+\) side).
Carbon tetrachloride has four polar covalent bonds. It is a tetrahedral molecule, so the charges are symmetrical.
The \(\delta+\) part of the molecule (the carbon atom) is locked away at the centre and no matter what direction you approach the molecule from you see a \(\delta-\) charge.
This means that the molecule itself is non-polar due to symmetry.