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QCD is the SU(3) gauge theory of the strong interaction. Quarks carry color charge (red, green, blue) and interact via eight massless gluons that themselves carry color. The QCD Lagrangian is structurally similar to QED but with three colors, eight gluons, and gluon self-interactions that produce qualitatively different physics.
Quantum chromodynamics is the theory of the strong interaction, built as an SU(3) Yang-Mills gauge theory. Quarks come in six flavors (up, down, strange, charm, bottom, top) and three colors (red, green, blue). Color is the charge of the strong force, analogous to electric charge in QED. The gauge bosons are eight gluons, each carrying one unit of color and one unit of anti-color. The QCD Lagrangian couples quarks to gluons through the covariant derivative, just as QED couples electrons to photons, but with SU(3) replacing U(1).
The crucial structural difference from QED is the self-interaction of gluons. Because gluons carry color charge, they interact with each other through three-gluon and four-gluon vertices. This has no analog in QED (photons are electrically neutral). The self-interaction makes QCD enormously richer: it produces asymptotic freedom (the coupling weakens at short distances), confinement (quarks cannot be isolated), and a complex vacuum structure. The gluon field contributes most of the proton mass (the quark masses account for only about 1% of the proton mass; the rest is gluon field energy and quark kinetic energy).
At high energies, asymptotic freedom means the strong coupling alpha_s is small and perturbative calculations are reliable. This regime is probed by deep inelastic scattering, jet production in collider experiments, and heavy quarkonium systems. The predictions of perturbative QCD — scaling violations in structure functions, the three-jet cross section at electron-positron colliders, the running of alpha_s — have been verified with percent-level precision.
At low energies (below about 1 GeV), alpha_s becomes large and confinement takes over. Quarks and gluons are permanently bound into color-neutral hadrons: mesons (quark-antiquark pairs) and baryons (three quarks). The mechanism of confinement is not fully understood analytically but has been confirmed by lattice QCD simulations, which show that the potential energy between a quark-antiquark pair grows linearly with separation. The transition from the perturbative to the non-perturbative regime — from quarks and gluons to protons and pions — is one of the central challenges of theoretical physics.