In water, oxygen shares a bond with two hydrogen atoms. oxygen is more than hydrogen, making the bond with the oxygen being the negative end and the two hydrogens being the positive end.

Oxygen is more ELECTRONEGATIVE than Hydrogen, which means that the electrons in the atom are coaxed more toward the Central Oxygen atom. Because of this, the charge is not evenly distributed among the molecule, causing it to have positive and negative sides. This is what we call a POLAR molecule.