Nucleon is the collective term for protons and neutrons. Nucleons are the particles found in the nucleus of atoms. Most notably nucleons are a result of the strong force holding the atoms together—which is stronger than the electric force pushing them apart (see figure 1). Through a beta decay, the weak force can turn one nucleon into another nucleon—either protons to neutrons or neutrons to protons. 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.