Quarks are fundamental elementary particles that constitute composite particles, such as protons and neutrons. Classified as fermions, they possess fractional electric charge and interact primarily via the strong nuclear force, which is mediated by gluons. The Standard Model of particle physics groups quarks into six known types, defined by their distinct electrical charges and color charges. These particles are not considered fundamental in the strictest sense by some theories, as they combine to form hadrons, which are the type of particles that make up atomic nuclei. Their existence and properties are central components of modern models describing the structure of matter.