The fourth force found in nature, but not in the nucleus, is the gravitational force. These forces are believed to be generated by the exchange of particles between the interacting pieces of matter. For example, the gravitational force is thought to be carried by particles called gravitons. The electromagnetic force is assumed to be exerted through the exchange of photons. The particles discovered have been classified into several categories.
Hadrons are particles that respond to the strong force and have internal structure.
Hadrons consist of Baryons and Mesons. A Baryon is composed of three quarks. A Meson is composed of a quark and an antiquark. Leptons are particles that do not respond to the strong force and have no internal structure. Quarks are particles with no internal structure that are thought to be the fundamental constituents of hadrons.