A quark is a tiny theoretical particle that makes up protons and neutrons in the atomic nucleus. Along with gluons, quarks also make up more exotic hadrons such as mesons, which are not stable. It is called theoretical because while assuming its existence makes for a better physics theory, it has never been observed directly.
Along with leptons — electrons, muons, the tau, and their associated neutrinos and antiparticles — quarks make up all the visible matter in the universe. They are the only fundamental particles which interact with each other through all four fundamental forces: strong nuclear force, weak nuclear force, electromagnetic force, and gravity. A basic property of these particles is confinement — all quarks make up hadrons, and are necessarily never independent. Descriptions of their physical properties emerged from quantum chromodynamics (QCD), the theory of the strong nuclear force which holds the atomic nucleus together.
Like all other subatomic particles, quarks may be described exhaustively by three quantum numbers: spin J, parity P, and mass m. Because they are never isolated, these properties must be inferred by observing the larger particles they make up. There are six known types: up, down, charm, strange, top, and bottom. These names are purely arbitrary and do not suggest anything about each quark's properties.
The normal matter that makes up the majority of the universe is made of up and down quarks, which are the lightest of the particles. A proton is made of two up and one down quark, while a neutron is made of two down and one up quark.
Quarks have varying masses, which are measured in GeV (giga electron-volts) over the speed of light squared. Subatomic particles are measured in terms of the energy they produce rather than mass in grams. The down quark is about twice as massive as the up. The strange is about 20 times as massive as the down quark. The charm quark is about 10 times as massive as that, followed by the bottom, which is about three times as massive as the last, and finally the top quark, which is the most massive of all. Increasing mass tends to correspond to scarcity of the particle, and necessitates more exotic physical conditions for its manifestation.
Physicists are on the lookout for theorized quark matter, a hypothetical lattice made of continuous quarks connected by gluons. It is not yet known whether this type of matter is physically possible. If so, it would probably be found in the core of extremely compact stars that have not yet collapsed into a black hole.