Illustrated Glossary of Organic Chemistry

Hydrogen bonding (H-bond): A noncovalent (van der Waals) attractive force caused by electrostatic attraction of a hydrogen atom with a lone pair of another atom. The hydrogen bond donor must have a sufficiently large δ+ charge caused by bonding to a highly electronegative element (O, N, or F; or in uncommon cases by strong electron-withdrawing inductive effects). The hydrogen bond acceptor must have a lone pair, and sufficiently high electron density (the accepting atom must have a negative formal charge, or if neutral must be oxygen or nitrogen).

General hydrogen bond structure.   
Hydrogen bonding in water.

Hydrogen bonding between fluoride ion and ethanol.   
Hydrogen bonding in the guanine-cytosine DNA base pair.