A gluon is the force carrying particle behind the strong force, the fundamental interaction that binds quarks together to form protons, neutrons, and other particles built from quarks. Without gluons holding quarks in place, ordinary atomic nuclei could never exist.
Gluons are unusual among force carriers because they themselves carry the very charge they transmit, called color charge. That means gluons can interact with other gluons directly, which makes the strong force behave very differently from electromagnetism at both very short and very long distances.
This self interaction is also why quarks and gluons are never observed alone in nature, a phenomenon called confinement, they only ever appear bound up together inside larger particles. Astrinova’s particle physics guide goes deeper into how the strong force works.
Physicists label the strong force’s charge using an analogy to color, calling the three types red, green, and blue, though it has nothing to do with actual visible color. Just as combining red, green, and blue light produces white, particles built from quarks must combine their color charges to produce an overall colorless, or white, result, which is part of why protons always contain exactly three quarks rather than some other number. Try to pull two quarks apart and the gluon field between them stretches like an elastic band, its energy growing the farther apart they get, until eventually it snaps and that energy converts into a brand new pair of quarks rather than letting either one escape alone. Particle collider experiments see the aftermath of this constantly, as sprays of particles called jets.
There are eight distinct types of gluon, a number that follows directly from the mathematics of how the three color charges can combine and recombine. Unlike the single, chargeless photon of electromagnetism, this larger, more tangled set of force carriers is part of why the strong force behaves so differently at short range and why quantum chromodynamics, the theory describing it, is mathematically far more complex than its electromagnetic counterpart.