Hydrogen Bonding

Molecules containing O-H and N-H bonds are polar and have permanent dipoles. 

These dipoles are strong

The permanent dipole-dipole interaction between the molecules containing either O-H or N-H bonds have a special name: a hydrogen bond.

In a hydrogen bond, the electron deficent Hδ + on one molecule attracts a lone pair of electrons on Oδ - or Nδ - on a different molecule.

Here is hydrogen bonding in a water molecule:

Thanks to hydrogen bonding an exception occurs, the solid property of the material become less dense than the liquid. Ice becomes less dense than water-

E.g. Ice can sit frozen on top of a lake.

This is because:

Ice has an open lattice with hydrogen bonds which holds the water molecules apart, meaning the weight is spread over a larger area

When Ice melts the rigid hydrogen bonds then collapse, allowing H2O molecules to move closer together, making them denser

The boiling and melting points of water are pretty high, this is caused by the strong hydrogen bonds between H2O molecules .

The hydrogen bonds act as extra forces over and above the Van der Waals' forces.

These extra forces then mean more energy is required to break the bonds in the H2O molecules, resulting in a high melting and boiling point than if water molecules just had the Van der Waals' force acting upon them.

The extra itermolecular bonding from the hydrogen bonds can also be used to explain the relatively high surface tension and viscosity (resistance) of water. 

E.g. When a small insect lands on water it does not sink as it is able to walk across a 'raft' of connected hydrogen bods.

As said before, hydrogen bonding occurs in O-H and N-H bonds, making it important in organic compounds. (alcohols, amino acids, carboxylic acids)

Hydrogen bonds are responsible for the shape of many molecules for example proteins

In DNA hydrogen bonds are also responsible for holding together the double helix structure.

There are 4 different bases in DNA: Guanine G, Cytosine C, Adenine A and Thymine T. The bases G and C, and A and T are held together by two hydrogen bonds.

Bases G and C, held together by 3 hydrogen bonds shown with a dashed line

Bases A and T held together by 2 hydrogen bonds shown with a dashed line