A hydrogen bond is the attraction between a hydrogen atom with a partial positive charge and another atom that has a partial negative charge. The receiving atom can be in a different molecule or a different part of the same molecule. To achieve the partial positive charge necessary for a hydrogen bond, the hydrogen atom must be bonded to an oxygen or nitrogen atom. The atom bonding to the hydrogen must be an oxygen, nitrogen, or fluorine with at least one unbonded electron pair. Hydrogen bonds are usually considered a particularly strong form of dipole-dipole interactions. The dipole created by a carbon-hydrogen bond is not strong enough to qualify as a hydrogen bond. A hydrogen bond is roughly 3-5% the strength of a covalent bond, and 5-20 times stronger than a typical dipole-dipole interaction.
Hydrogen bonding plays a key role in biochemistry as it influences the shape and arrangement of large molecules. The double stranded nature of DNA, as well as its proper pairing of bases is determined almost entirely by hydrogen bonds. Hydrogen bonding is critical to proper protein folding.