H2O is a polar covalent molecule. Polar covalent molecules occur when heterogeneous molecules occur between atoms of different electro-negativity (E.N.), which is a measure of how strongly an atom attracts a shared pair of electrons in a bond. For example the E.N. of H is 2.1 and the E.N of O is 3.5 so in H2O, O is slightly negative relative to H and H are slightly positive, relative to O. Therefore the electron clouds are not symmetrical and electrons are attracted more to O. Here is a picture. Opposite ends of a dipole attract by electrostatic attraction. Electrostatic attraction between adjacent molecules is a physical, not chemical property. Polarity affects forces between molecules. The more polar molecules are, the harder they are to separate. This affects the melting and boiling points of molecular substances. See this picture. H2O is more polar, so attraction between H2O molecules is stronger (bigger δ+ and δ- charges). Because H2O is very polar, these forces are strong and H2O is a liquid at room temperature, while H2S is not. See this picture. The bonds between H2O molecules are hydrogen bonds. Not a chemical bond, but a strong attraction between the H atom δ+ dipole of molecule and the δ- dipole (in this case O) of an adjacent molecule. Hope that helps.