Hadrons are particles which are held together by their strong interaction. Hadrons include mesons and baryons but excludes leptons, which do not interact by the strong force. The weak interaction acts on both hadrons and leptons.
Hadrons consist of quarks, either as quark-antiquark pairs (mesons) or as three quarks (baryons). Quarks are surrounded by a cloud of gluons, the exchange particles for the color force. Recent experimental evidence has showed the existence of five-quark combinations that are being called pentaquarks.
Hadrons are assigned quantum numbers which correspond to the representations of the Poincaré group: JPC(m), where J is the spin quantum number, P, the intrinsic (or P) parity, and C, the charge conjugation, or C parity, and the particle four-momentum, m.