A polar bond is defined as a type of covalent bond between two atoms in which electrons are shared unequally. The unequal sharing of electrons is due to electronegativity differences. Electronegativity is defined as the measure of the tendency of an atom to attract electrons. Specifically, an electronegativity difference between the atoms involved in a bond that ranges between .3 to 1. results in a polar covalent bond. Electrons migrate towards the atom with a greater electronegativity. This results in the atom with the greater electronegativity having a slightly negative charge and the atom with the lower electronegativity having a slightly positive charge.
Thus, water is polar because the electronegativity differences between hydrogen and oxygen is within this range. Hydrogen has an electronegativity of 2.1 whereas oxygen has a value of 3.5, resulting in an overall difference of 1.4. Thus, the electrons are pulled closer to the oxygen than they are the hydrogen. This causes the oxygen to have a slight negative charge and the hydrogens to have slight positive charges. The link below provides a picture of such.