A polar molecule is characterized by the uneven distribution of the electrons that form the covalent bonds between each atom in the molecule, resulting in a slightly positively charged side and a slightly negatively charged side. This occurs because of the differences in electronegativity between atoms of different elements.
Water, or H2O, is an example of a polar molecule. The oxygen atom in the water molecule has a greater electronegativity than the hydrogen atoms it is covalently bonded to, resulting in a dipole shift where the bond is negatively weighted on the oxygen end and positively weighted the hydrogen end. The shift in electrical charges within the molecule is subtle and results in an overall electrically balanced structure with a nonlinear bent shape. The shape of the molecule is just as important as the charges associated with each end when it comes to determining whether a molecule is polar or not.
In the case of carbon dioxide, or CO2, the charges are unevenly distributed between the oxygen atom and the carbon atoms, but the molecule is in a linear shape, so the dipole shifts balance each other out on either end and result in a non-polar molecule. Intermolecular interactions can occur between polar molecules due to the dipole shifts and slight charges associated with each end. In the case of water, the negatively charged end of one water molecule interacts with other water molecules by weakly attracting the positively charged ends and repelling the negatively charged ends.