Hydrogen bonding occurs when hydrogen is bound to a strongly electronegative element, O or F or N. The heteroatom polarizes electron density towards itself, and acquires a partial negative charge. Conversely, the hydrogen acquires a partial positive charge because the heteroatom has denuded it (slightly!) of electron density. This is, in a nutshell, the phenomenon of hydrogen bonding, which is responsible for the exceptionally high boiling points of OH_2, and HF, and NH_3.
Thus between molecules, there is an additional electrostatic interaction. The dipoles line up, positive to negative. Ethylene glycol, HOH_2C-CH_2OH has two polar hydroxyl groups, and the dipoles line up from molecule to molecule. This constitutes a potent intermolecular force, which accounts for the high boiling point of ethylene glycol, 197.3 ""^@C, and also its viscosity.
