The Standard Model
The standard model explains all observations so far made by particle physicists. The Standard Model consists of elementary particles grouped into two classes: bosons (particles that transmit forces) and fermions (particles that make up matter). Bosons have particle spin that is either 0, 1 or 2. Fermions have spin 1/2.
The table above lists the elementary particles in the Standard Model that transmit the four forces observed in Nature, though the gravition is not actually part of the Standard Model.
The evidence for quarks is indirect as they have not been directly observed. They are believed to be confined inside hadrons and unobservable as single particles, except possibly at extremely high temperatures such as could be found very early in the Big Bang.
The fermions in the Standard Model are particles that make up matter, and are grouped into three generations . Notice that the quarks with charge 2/3 come in a group of three, as do the quarks with charge -1/3, as do the electron, muon and tau, and the electron, muon and tau neutrinos. In each group, the heavier particles are shown in the larger type.
It is not yet known why there are three generations of particles that make up matter.