A hydrogen bond is an electrostatic interaction between two polar groups that occurs when a hydrogen (H) atom covalently bound to a highly electronegative atom such as nitrogen (N) or oxygen (O) experiences the electrostatic field of another highly electronegative atom nearby.
Hydrogen bonds can occur between molecules (intermolecular) or within different parts of a single molecule (intramolecular). Depending on geometry and environment, the hydrogen bond free energy content is between 1 and 5 kcal/mole. This makes it stronger than a van der Waals interaction, but weaker than covalent or ionic bonds. This type of bond can occur in inorganic molecules such as water and in organic molecules like DNA and proteins.