A quark is an elementary particle with an electric charge. When quarks combine, they form another type of particle called a "hadron." If you want to learn more, take up particle physics.
Quarks are held together by what physicists call "the strong force" or "the strong nuclear force." The most stable of these quark combinations, or hadrons, are protons and neutrons — the basic building blocks of atoms. In the 1960s, a physicist named Murray Gell-Mann named these particles from a favorite made-up word in James Joyce's book "Finnegan's Wake," quark. Gell-Mann's partner reportedly preferred the word ace.