Nucleon is the collective term for protons and neutrons. Nucleons are the particles found in the nucleus of atoms. Most notably nucleons are subject to the strong force which holds them together, which is stronger than the electric force pushing them apart (see figure 1). The weak force can turn one nucleon into another nucleon, either protons to neutrons or neutrons to protons, through beta decay. Nucleons are incredibly small, about 10-15 m, 10,000x smaller than an atom! Please see size of the universe for some online demonstrations to show this scale.
To learn more about neutrons please see hyperphysics.