What are intermolecular forces?
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. Molecules are usually attracted to each other over a wide range of distances, but once they get very close to each other they will be repelled from each other (more details).
Intermolecular forces are important in determining the state of matter a substance adopts at a specified temperature and pressure. In a solid the average 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 average 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 average kinetic energy of the molecules is much greater than the intermolecular forces between them and the molecules move freely, colliding less frequently than in a liquid and with much greater distances between them. In fact most of the volume taken up by a gas is empty space.