A quark is a fundamental particle and a fundamental constituent of matter. It is one of the building blocks of protons and neutrons, which are subatomic particles found in the nucleus of atoms. Quarks are elementary particles, meaning they have no known substructure or smaller components. 
Quarks come in different types, or “flavors,” known as up, down, charm, strange, top, and bottom. Each flavor carries a fractional electric charge and has different properties such as mass and stability. Up and down quarks, for example, are the lightest and most stable quarks and are the ones primarily found in everyday matter.
Quarks have a property called “color charge,” which is unrelated to visual color. The color charge describes the strong nuclear force that holds quarks together via the exchange of particles called gluons. Quarks experience the strong force and the weak force, two of the fundamental forces in nature.
Quarks are never observed in isolation due to a phenomenon called confinement. They are always found in combinations of two or three, forming composite particles called hadrons. Protons and neutrons, for instance, are made up of combinations of up and down quarks.
Our understanding of quarks and their interactions is based on the theoretical framework of quantum chromodynamics (QCD), which is part of the Standard Model of particle physics. Quarks and their properties are actively studied through experiments conducted at particle accelerators and through theoretical research in the field of high-energy physics.
