Polar molecules are molecules that have a partial positive charge on one side and a partial negative charge on the other. This occurs due to differences in electronegativity between the atoms involved, causing an uneven distribution of electron density. Water (H2O) is a common example of a polar molecule, where the oxygen atom attracts electrons more strongly than the hydrogen atoms. The polarity of molecules affects their interactions with other substances. Polar molecules tend to dissolve well in polar solvents, such as water, while they do not mix well with nonpolar substances like oils. This property is crucial in biological systems and chemical reactions.