Alongside gravity, electromagnetism and the weak nuclear force, the strong nuclear force is one of the four fundamental forces in the universe. Only fundamental particles (e.g., quarks and gluons) are believed to be affected by the strong nuclear force. The strong nuclear force holds quarks together to form baryonic particles; a baryon is any particle made up of three quarks. The strong nuclear force operates similar to an elastic band: the further apart quarks held together get, the stronger the force.
Quarks are fundamental particles that have a spin, charge and a "color" (the color for a quark is not an actual color, but is instead a description of how it responds to the strong nuclear force). Additionally, there are six types of quarks: up, down, charm, strange, top and bottom. Quarks either combine in threes to form baryons or they can combine with an antiquark to create a meson; mesons disappear almost as quickly as they are created.
Due to their infinitesimally small size, quarks are difficult to visualize; however, they do form two important particles found in all atoms: protons and neutrons (electrons are themselves fundamental particles, and thus are not made up of quarks). Protons are the defining characteristics of an atom, as the number of protons is what type of element it is (e.g., all oxygen atoms have eight protons, and if a new proton was added, it would be a fluorine atom). Neutrons combine with protons to make up the nucleus of an atom, which is where most of the atom's mass is located.
Protons were discovered before quarks; credit for the discovery of protons goes to Ernest Rutherford, who performed an experiment in 1909 that showed atoms have a centralized mass. Rutherford coined the term "proton" for the particle that carries this mass. Quarks were later theorized by Murray Gell-Mann in the 1960s, and verified by subsequent experiments. Gell-Mann won the 1969 Nobel Prize for his discovery. The name "quark" comes from James Joyce's "Finnegan's Wake."