This document discusses different types of intermolecular forces (IMFs) between molecules. It defines IMFs as interactions between molecules due to polarity, as opposed to intramolecular bonds within a molecule. The four main types of IMFs are described from weakest to strongest: dispersion forces, dipole-dipole forces, hydrogen bonding, and ion-dipole forces. Hydrogen bonding is highlighted as a particularly strong type of dipole-dipole force that occurs between polar molecules containing hydrogen bonded to oxygen, nitrogen, or fluorine.