The fundamental particles that make up protons and neutrons are called quarks. Protons and neutrons, collectively known as nucleons, are found in an atom's nucleus. Quarks combine in groups to form these nucleons; protons consist of two up quarks and one down quark, while neutrons consist of one up quark and two down quarks. Quarks are held together by the strong nuclear force, mediated by gluons. Understanding quarks and their interactions is vital in the field of particle physics.