London dispersion forces are a part of Van Der Waal's forces acting between atoms and molecules. They are a temporary attractive force that are formed when two atoms close to each other form a temporary dipole and are hence the weakest intermolecular force of attraction. They are usually the force of attraction found between non-polar molecules.
While a hydrogen bonding a strong intermolecular force (dipole-dipole) of attraction formed as a result of the interaction between an electronegative atom (like oxygen or chlorine) and a hydrogen atom bonded to another electronegative atom (like fluorine). Since, molecules that posses hydrogen bonding have stronger intermolecular forces, they thus possess higher melting point than molecules that possess only London dispersion forces.
