A quark is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons, such as baryons (of which protons and neutrons are examples) and mesons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.

A proton is made of two up-quarks and one down-quark, while a neutron is made from one up-quark and two down-quarks. Electrons aren't supposedly made from any quarks, and are at their smallest already.