Hydrogen bonding is a type of dipole-dipole interaction (dipole being when a molecule has unequal sharing of electrons across a bond). It is a particularly strong type dipole-dipole interaction and can only occur between oxygen/fluorine/nitrogen and hydrogen due to the large difference in electronegativity between these atoms (electronegativity being a measure of how strong an affinity some atom has for a bonding pair of electrons). For example in a water molecule the oxygen atom attracts the shared pair of electrons to itself, away from the hydrogen, as oxygen has the greater electronegativity. This results in the oxygen atom having a slight negative while the hydrogen atom slight positive charge. As a result hydrogen bonding can effect properties such as the solubility of molecules in water or other solvents.