Hydrogen bonding is the intermolecular forces which occur between a hydrogen atom which is bonded to a highly electronegative atom and another highly electronegative atom.
Generally, only three elements are considered highly electronegative and they are fluorine, oxygen and nitrogen (FON).
It is the strongest intermolecular force of attraction due to the large differences in electronegativities between the hydrogen and FON. This leads to very polar molecules and hence strong electrostatic forces of attraction.