So far we have met the particles that make up most of the matter we see all around us - the up quark, the down quark and the electron. We have also met the electron neutrino, which is emitted during radioactive decay. It seems like these four particles, along with the particles that carry forces, are enough to explain everything.
It turns out that each of the matter particles has two "big brothers" - new particles that are identical except for their larger mass. Physicists talk about "three generations" (sometimes called families instead) of matter. The first generation is the particles we have met already. The second generation contains the charm quark (the big brother of the up quark), the strange quark (the big brother of the down quark), the muon (the big brother of the electron) and the muon neutrino (the big brother of the electron neutrino). The third generation is the top quark, the bottom quark (this is sometimes also called the beauty quark), the tau lepton and the tau neutrino.