The gluon is the carrier particle of the strong force that holds quarks together. Gluons are electrically neutral and have and spin 1. They are the equivalent in quantum chromodynamics (QCD) of photons in quantum electrodynamics (QED). Neither quarks nor gluons exist as isolated particles, but only as components of hadrons.
Associated with gluons is something called color charge, which is analogous to the electrical charge of particles that experience the electromagnetic force. However, the fact that the carrier of the strong force carries color charge so that it is itself supject to the strong force makes QCD much more complex, mathematically, than QED.
Unlike the single photon of QED or the three W and Z bosons that mediate the weak force, there are eight distinct types of gluon in QCD.