Intermolecular forces are the attractive or repulsive forces between molecules, essential for understanding the physical properties of substances. They include van der Waals forces, dipole-dipole interactions, and hydrogen bonds. Van der Waals forces occur between all molecules, dipole-dipole interactions happen between polar molecules, and hydrogen bonds are strong attractions involving hydrogen bonded to electronegative atoms like oxygen or nitrogen. These forces influence boiling points, melting points, and solubility.