What is a Hydrogen bond?
Hydrogen bonds are not really bonds but intermolecular forces - weak forces which arise between molecules.
Hydrogen bonds are the strongest kind of intermolecular force, but are still much weaker than the covalent bonds that hold the atoms together inside a molecule - remember the intermolecular forces operate from one molecule to another.
To form a hydrogen bond we need molecules which contain a hydrogen atom which is bonded to one of the most electronegative elements (N, O or F). As a result of the electronegativity the hydrogen atom will have a partial positive charge and will therefore be electrostatically attracted to the lone pair of electrons on one of these electronegative atoms on a nearby molecule.
Hydrogen bonds formed in this way are responsible for some surprising properties: water has hydrogen bonding, which is why pond-skater insects can walk on its surface, why you can place about 40 drops of water on a single small coin (try it!), why ice floats when being solid we'd expect it to be more dense than water and sink, and why water has a high boiling point compared with the hydrides of other Group 6 elements.