Atoms can sometimes combine to form molecules or pairs of ions. When molecules form, the atoms are covalently bonded by shared pairs of electrons. If the atoms have unequal attraction for the shared electrons, the atom with greater electronegativity will hold more negative charge. This results in a polar bond. 

The electronegativity difference between atoms is related to the excess bond energy from the attraction of opposite partial charges. Water (H2O) has an oxygen atom with electronegativity 3.4 and hydrogen atoms with electronegativity 2.2. The hydrogen-oxygen bonds are polar, with partial positive charge on hydrogen and partial negative charge on oxygen. Since water has a bent, non-linear shape, the bond dipoles combine vectorially to give a net molecular dipole. We say water is a polar molecule.

This has two consequences. First, water can dissolve ionic and polar compounds. Second, pure water can spontaneously ionize slightly to produce hydronium (H3O+) and hydroxide (OH-) ions, each typically around 10^-7 molar concentration.