First of all, there are actually 6 kinds or flavours as they are usually called, of quarks: up, down, top, bottom, strange and charm quarks. They are part of the elementary or base particles in the Standard Model, a table of sixteen elements which incorporate the mass, spin, electromagnetic force and name. The only force that quarks have that is not represented by the Standard Model would be color charge which will be explained later. Quarks are classified in the top left corner of the table and they belong to a family of particles called fermions. They are actually classified in three generations or pairs, which are the first generation up and down quarks, the second generation strange and charm quarks and the third generation top and bottom quark. Following the discovery of these there have been attempts at finding a fourth generation but at this time all attempts have failed. One of the qualities that puts the quark in the fermions’ is it’s spin, because all quarks are spin-1/2 particles, which classifies it as a fermion according to the spin statistics theorem. Quarks are the only particles that have all of the fundamental interactions or forces which include the afore said electromagnetism plus gravitation, strong interaction and weak interaction. Above all however, the main characteristic of quarks is that they are the only particles which do not have integer, that is to say whole number multiples, electric charges. Any of the quarks is also able change into the other quarks in its generation by a process called particle degeneration which is where the mass of one of the quark changes thus transforming it into its partnering quark. Of course everything must have an opposite and the same is true with quarks because there are also anti-quarks which are almost identical to regular quarks but have the opposite electric charge. The combinations that quarks and anti-quarks produce are either called hadrons, mesons or baryons depending on their proportions. Protons and neutrons, the most stable hadrons, are formed of 2 up quarks and 1 down quark or 1 up quark and 2 down quarks respectively. Baryons are made up of 3 quarks, thus the anti-baryon is made up of 3 anti-quarks, and mesons are made of one regular quark and one anti-quark, usually the same flavor as the regular one. There have also been experiments to determine if there might be other «exotic» hadrons which would have up to four of five valence quarks but to this day there has been no proof of their existence. History of the quark:
In the beginning no one knew that quarks actually existed. They simply came into existence, as a term, for explaining something they did not know of. At that time the particle zoo, that is to say the know particles at the time, consisted mostly of what we now call hadrons so they would have been particles similar to the proton and neutron. Then there came along a certain Murray Gell-Mann and his partner George Zweig who proposed that the particles that they did know of were not the smallest that there were but rather that each of these were made up of smaller ones. The term they used to explain these smaller particles were, indeed, up, down and strange quarks, although they did not have any evidence of their existence. The correct term for what they used quarks for would be an «abstract representation». The year after this proposal Sheldon Lee Glashow and James Bjorken suggested a forth flavor of quarks called charm to better explain the weak interaction between the particle, causing particle decay. The discovery of this new quark made it so that the number of quarks they knew of equalled the number of leptons they knew existed plus it lead to a formula for calculating mass which correctly reproduced the same masses as those of the known mesons. Then in 1961, Murray Gell-Mann and George Zweig came back with a particle...