A polar molecule is a particle consisting of two or more covalently bonded atoms with an asymmetric distribution of charges. This occurs in molecules that are asymmetric along at least one axis when one side contains atoms with a greater electronegativity than the other side. This results in shared electrons spending more time on the side with greater electronegativity, causing that side to have a greater negative charge most of the time.
One of the most familiar and common polar molecules on Earth is water. In water, two hydrogen atoms are bonded to one oxygen atom. However, the hydrogen atoms bond at an angle less than 180 degrees, resulting in an oxygen side and a hydrogen side of each molecule. The oxygen atom has a greater electronegativity than the hydrogen atoms and so receives a greater share of the molecule's shared electrons. This means that the hydrogen side is more positively charged than the oxygen side. Water molecules in close proximity will tend to align next to each other with each oxygen side facing the hydrogen side of another because of these opposite charges. Any polar molecule will tend to act in the same way. When water or a similar polar compound freezes, the crystal lattice it forms always tends to give maximal exposure of each positive side to adjacent negative sides.