the generations of matter
Nature seems to have a stutter. Every matter particle you are made of, the up quark, the down quark, the electron, the electron neutrino, has two heavier duplicates that are identical in every way except mass. Stack them up and the fundamental fermions fall into three neat columns, called generations or families. It is one of the strangest facts in physics: the universe apparently only needs the first generation to build atoms and stars, yet it comes supplied with two spare copies that flash into existence in accelerators and cosmic rays and then decay away.
A generation is a set of four fermions with identical gauge quantum numbers: an up-type quark, a down-type quark, a charged lepton and its neutrino. Generation one is (up, down, electron, electron-neutrino); generation two is (charm, strange, muon, muon-neutrino); generation three is (top, bottom, tau, tau-neutrino). The only difference between generations is mass, and the mass jumps are enormous, the top quark being some 75,000 times heavier than the up. Because the heavier generations are unstable, they cascade down to the first, which is why ordinary stable matter uses only the lightest column.
How many generations are there? Experiment gives a sharp answer for light neutrinos: the measured decay width of the Z boson at LEP matches exactly three neutrino species, so there is no fourth generation with a light neutrino. Why three, and why the wild spread of masses, is completely unexplained; it is dialed in by hand through the Yukawa couplings to the Higgs field. The generation structure is also the stage on which quark mixing (the CKM matrix) and CP violation play out, tying this puzzle to the deeper mystery of why the universe is made of matter.
The muon and the tau are, quantum-number for quantum-number, just heavy electrons; a tau (generation three) can decay into a muon (generation two) which decays into an electron (generation one), a literal cascade down the ladder of families.
Heavier generations always decay toward the lightest, which is why everyday matter is built from generation one alone.
The count of exactly three generations of light neutrinos is an experimental result from the Z boson lineshape, not a prediction of the Standard Model, which would happily accommodate more.