The quark model is a theory in particle physics that describes the fundamental building blocks of matter. According to this model, protons and neutrons, which make up atomic nuclei, are not elementary particles but are composed of smaller particles called quarks. There are six types of quarks, known as "flavors": up, down, charm, strange, top, and bottom. Quarks combine in groups to form larger particles called hadrons. The most common hadrons are baryons, which include protons and neutrons, and mesons, which are made of a quark and an antiquark. The quark model helps explain the properties and interactions of these particles in the universe.