What is the difference between covalent bonds, and hydrogen bonds?

The covalent bond formed between oxygen and hydrogen in water, or hydrogen and fluorine in hydrogen fluoride, or nitrogen and hydrogen in ammonia, or hydrogen and chlorine in hydrogen chloride, is CLEARLY an #"INTRAMOLECULAR phenomenon"#, and this gives rise to discrete molecules of water, ammonia or hydrogen fluoride/chloride.

But when hydrogen is bound to these strongly electronegative elements, i.e. oxygen or nitrogen or an halogen, charge separation occurs, and the electronegative element polarizes electron density towards itself. We could represent this by the diagrams....

#stackrel(delta+)H-stackrel(delta-)O-stackrel(delta+)H#; or #stackrel(delta+)H-stackrel(delta-)F#; or #stackrel(delta+)H""_3stackrel(delta-)N#.

When these dipoles line up in solution this constitutes a potent intermolecular force, and is responsible for the elevated boiling points of these hydrides compared to the lower group hydrides, where the electronegativity difference is not so pronounced.

Hydrogen bonding thus occurs as AN INTERMOLECULAR phenomenon to some extent in #H-Cl#, but the dipole is not so strong as in #HF#, and as a consequence the normal boiling point is lower (you should look these up pronto!).

And so the answer in a nutshell, covalent bonds are INTRAMOLECULAR, i.e. within a molecule. If the covalent bond is strongly polarized, i.e. we gots #H-O#, or #H-F#, or #H-N#, where hydrogen is bound to a strongly electronegative element, charge separation and polarity can occur, with INTERMOLECULAR HYDROGEN BONDING as a consequence.

Capisce?