Quarks never exist alone but only in groups. Quarks are differentiated from leptons, the other family of elemental particles, by electric charge. Leptons? have integral charge (+1, 0 or -1) while quarks have +2/3 or -1/3 charge (antiquarks have -2/3 or +1/3 charge).
The six quarks are known as:
A proton is made of two up quarks and one down, giving a charge of +1. A neutron is made of two down and one up, giving a charge of zero. Only up and down quarks are needed for ordinary matter; the other quarks degenerate quickly into those two types.
According to the theory of [Quantum Chromodynamics]? quarks possess another property that is called "color charge" (and that doesn't have anything to do with real color). Instead of just two different charge types (like + and - in electromagnetism), color charge comes in 3 types: "red", "green" and "blue" (6 if we count the "anticharges"). In the theory, only "color neutral" particles can exist, in that way, a baryon (like a neutron or a proton) is made up of one red, one blue and one green quark. Particles of different color charge are attracted and particles of like color charge are repelled by the strong nuclear force, which is transfered by gluons, particles that themselves carry color charge. Therefore, colors of quarks are not static, but are interchanged by gluons, always maintaining the result neutral. This interchange of color charge keeps quarks together forming a baryon, or meson, and a "secondary" effect of this strong nuclear force is to keep the nucleons together!
There is a second kind of particle composed by quarks called mesons. Mesons are formed by quark-antiquark configurations where the particle is color neutral because of the color-anticolor presence (so you don't need three quarks, you can have just two, one of a color and the other of "anticolor".
Due to the extremely strong nature of the [strong force]? (hence its name), quarks are never found free. They are always bound into baryons, mesons, or nuclei. When we try to separate quarks in a meson or baryon, as happens in [particle accelerators]?, the strong force actually becomes stronger as they get farter apart. At some point it is more energetically favorable to create two more quarks to cancel out the increasing force, and nature pops two new quarks out of the vacuum. This process is called hadronization? or fragmentation, and is one of the least understood processes in particle physics. As a result of fragmentation, when quarks are produced in particle accelerators, instead of seeing the individual quarks in detectors, scientists see "jets" of many color-neutral particles (mesons and baryons), clustered together.
The theory behind quarks was first suggested by physicists Murray Gell-Mann and George Zweig, who found they could explain the properties of many particles by considering them to be composed of these elementary quarks. The name quark comes from "three quarks for Muster Mark"", a nonsense phrase in James Joyce's Finnegan's Wake
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