A quark is a fundamental particle and a basic building block of matter. They combine to form protons and neutrons, which are found in the nucleus of an atom. Quarks come in six different types, known as "flavors": up, down, charm, strange, top, and bottom. Each flavor has unique properties, such as mass and charge. Quarks are held together by the strong force, which is mediated by particles called gluons. This interaction is described by the theory of quantum chromodynamics. Quarks are never found in isolation; they always exist in groups, forming composite particles called hadrons, such as protons and neutrons.