These smallest particles fall into several main categories, most notably fermions, hadrons and bosons. Fermions are the building-block particles. There are two types of material fermions: quarks, which work to hold the nucleus of an atom together, and leptons, which do not. Fermions can be broken down even further: There are different types of quarks, and for each, an antiquark. Quarks are found in groupings, but leptons are found alone. Electrons and neutrinos are examples of leptons. Fermions have a half-integer spin [source: University of California Davis].