A substance comprised of molecules that contain unbalanced localized charges (dipoles) is a polar substance. Polar substances tend to interact with other polar substances and rarely react to a significant degree with nonpolar substances.
When two atoms form a covalent bond, they each share an electron. Different atoms have different abilities to attract electrons quantified by their electronegativity values. When one atom in a covalent bond has a much higher electronegativity than the other, the electron is found closer to that atom than the weaker one. This creates a dipole effect with a slight negative charge at the atom the electron favors and a slight positive charge at the other.