Intermolecular forces are forces that keep molecules together. For example, the forces between two water molecules. The stronger the intermolecular forces are, the more "solid" is the matter going to be, meaning that the intermolecular forces are the strongest in solids and weakest in gases. Make sure not to confuse intERmolecular forces (forces between *molecules*) and intRAmolecular forces (forces between *atoms* that make up a molecule).
Protons and neutrons are all attracted to each other as a result - the strong nuclear force. This is an attractive force that only has an effect over a very short range in the nucleus.
