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The quark model classifies all observed hadrons as bound states of quarks: mesons are quark-antiquark pairs and baryons are three-quark states. Combining quark flavors, spins, and orbital angular momenta using SU(3) flavor symmetry and angular momentum addition rules reproduces the observed hadron spectrum, including the pseudoscalar and vector meson octets, the baryon octet and decuplet, and their excited states.
The quark model, developed by Gell-Mann and Zweig in 1964, organizes all known hadrons as composite states of quarks. Mesons are quark-antiquark (qqbar) pairs and baryons are three-quark (qqq) states, with each state required to be a color singlet under SU(3)_C. The quantum numbers of a hadron -- spin, parity, charge conjugation, isospin, strangeness -- follow from combining the quantum numbers of its constituent quarks using standard angular momentum addition rules.
The light hadron spectrum is organized by SU(3) flavor symmetry, which treats the up, down, and strange quarks as an approximate triplet. Mesons (qqbar) decompose as 3 x 3-bar = 8 + 1, giving octets and singlets. Baryons (qqq) decompose as 3 x 3 x 3 = 10 + 8 + 8 + 1, and the ground-state baryons fill the spin-1/2 octet (proton, neutron, Sigma, Xi, Lambda) and the spin-3/2 decuplet (Delta, Sigma*, Xi*, Omega-). The prediction and subsequent discovery of the Omega- baryon (sss, spin 3/2) in 1964 was a dramatic confirmation of the model.
Mass splittings within and between multiplets arise from two sources: the quark mass differences (m_s >> m_u, m_d breaks SU(3) flavor symmetry) and the color-magnetic hyperfine interaction (the spin-spin interaction between quarks mediated by one-gluon exchange). The hyperfine interaction explains why the Delta is heavier than the nucleon, why the rho is heavier than the pion, and why the Sigma is heavier than the Lambda -- in each case, the state with aligned quark spins is heavier. These splittings provide a window into the QCD dynamics inside hadrons.
Excited hadrons with nonzero orbital angular momentum L fill out additional multiplets: the L=1 mesons include the a_1, b_1, f_1 states, while L=1 baryons include the N(1520) and N(1535) resonances. The Regge trajectories -- plots of spin J versus mass-squared M2 -- show remarkably linear relationships, reflecting the string-like behavior of the confining flux tube between quarks. While the quark model is not a rigorous derivation from QCD (it uses constituent quarks with effective masses of ~300 MeV, much larger than the current quark masses), it remains an indispensable organizing principle for hadron physics.