Question

Define hydrogen bond. Is it weaker or stronger than the van der Waals forces?

Answer 1

A hydrogen bond is defined as an attractive force acting between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule (may be of the same kind). 
Due to a difference between electronegativities, the bond pair between hydrogen and the electronegative atom gets drifted far away from the hydrogen atom. As a result, a hydrogen atom becomes electropositive with respect to the other atom and acquires a positive charge.

$4^δ-X^{δ-}.......H^{δ+} -X^{δ-}.....H^{δ+}-X^{δ-}$

The magnitude of $H-bonding$ is maximum in the solid state and minimum in the gaseous state. 
There are two types of $H-bonds$ : 
(i) Intermolecular $H-bond$ e.g., $HF$, $H_2O$ etc.
(ii) Intramolecular $H-bond$ e.g., $o-nitrophenol$

Hydrogen bonds are stronger than Van der Walls forces since hydrogen bonds are regarded as an extreme form of $dipole-dipole$ interaction