What are hydrogen bonds? what type of molecules do hydrogen bonds occur in?

A hydrogen bond is a type of attractive intermolecular force that exists between two partial electric charges of opposite polarity. Although stronger than most other intermolecular forces, the hydrogen bond is much weaker than both the ionic bond and the covalent bond.

They occur in inorganic molecules like water, and organic molecules like DNA and proteins.

Hydrogen bond is formed only by the three highly electronegative elements- fluorine, oxygen and nitrogen. So, hydrogen bonding is possible only in those compounds in which the hydrogen atom is directly bonded to fluorine, oxygen or nitrogen.