Before the Standard Model (SM) was developed in the 1970s (the key elements of the Standard Model known as quarks were proposed by Gell-Mann and Zweig in 1964), physicists observed hundreds of different kinds of particles in particle accelerators. These were organized into relationships on their physical properties in a largely ad-hoc system of hierarchies, not entirely unlike the way taxonomy grouped animals based on their physical features. Not surprisingly, the huge number of particles was referred to as the "particle zoo".

The Standard Model, which is now the prevailing model of particle physics, dramatically simplified this picture by showing that most of the observed particles were mesons, which are combinations of two quarks, or baryons which are combinations of three quarks, plus a handful of other particles. The particles being seen in the ever-more-powerful accelerators were, according to the theory, typically nothing more than combinations of these quarks.

Within the Standard Model, there are several different types of particles. One of these, the quarks, has six different kinds, of which there are three varieties in each (dubbed "colors", red, green, and blue, giving rise to QCD: quantum chromodynamics). Additionally, there are six different types of what are known as leptons. Of these six leptons, there are three charged particles: the electron, muon, and tauon. The neutrinos comprise the other three leptons, and for each neutrino there is a corresponding member from the other set of three leptons. In the Standard Model, there are also bosons, including the photons, W+, W-, and Z particles, gluons, and a few open spaces left for the graviton and Higgs boson, which have not yet been discovered. Almost all of these particles come in "left-handed" and "right-handed" versions (see chirality). The quarks, leptons and W boson all have antiparticles with opposite electric charge.