Quantum chromodynamics (QCD) is the theory that describes the strong nuclear force, which is one of the four fundamental forces in nature that holds quarks together to form protons, neutrons, and other particles. QCD is based on the principles of quantum mechanics and the theory of colors, with quarks being assigned three color charges (red, green, and blue) and gluons being the force carriers that mediate interactions between quarks. The study of QCD involves understanding the behavior of quarks and gluons within the framework of quantum field theory, and has implications for particle physics, nuclear physics, and the early universe.