One important thing that we should know is that polarity has to do with the presence of a resultant dipole moment in a molecule.
Dipole moment is a vector quantity, This means that its direction is also taken into account when discussing the dipole moment of molecules.
Hence, symmetrical molecules such as CS2 and CCl4 are non-polar even though they have polar bonds because their dipoles cancel out(zero resultant dipole moment).
On the other hand, NH3 and CH3OH are non-symmetrical molecules hence they possess an overall dipole moment and are polar molecules.
