Intermolecular forces are forces of attraction or repulsion between molecules.
The size and direction of the force between a pair of molecules depends on the distance between the molecules and sometimes on the relative orientation of the molecules. We study intermolecular forces because they are important in determining the state of matter a substance adopts at a specified temperature and pressure. In a solid the kinetic energy of the molecules is small compared to the strength of the intermolecular forces, so each molecule can only move short distances around a fixed position. In a liquid the kinetic energy of the molecules is comparable to the intermolecular forces between them, so the molecules can move around, but usually stay within a molecular diameter of each other. In a gas the kinetic energy of the molecules is much greater than the intermolecular forces between them and the molecules move freely, colliding far less frequently than in a liquid.