An uneven distribution of electron density makes water molecules polar. Within each molecule, there is a partial negative charge near the oxygen atom because of an unshared pair electrons and partial positive charges near the hydrogen atoms.
Each of the two hydrogen atoms in a water molecule shares a covalent bond with the single oxygen atom in the molecule. The oxygen atom also has two pairs of electrons that are unshared, bringing the total number of electron pairs surrounding the oxygen atom in a water molecule to four. The two pairs shared with the hydrogen atoms remain on one side of the oxygen atom, and the unshared electrons remain on the opposite side. This helps to create the molecule's polarity.
Polarity gives water molecules many of its special properties. Because of its polarity, water acts as an excellent solvent. Other polar molecules, like sodium chloride, dissolve in water so readily that water is often referred to as the universal solvent. This also causes pure water to be a rare thing in nature.
Polarity also causes water to be unable to combine with non-polar molecules. A common example of this is oil not combining with water. Oil, fats and other non-polar molecules are known as hydrophobic, or water fearing, molecules because of this property.