Part of the Standard Model of Particle Physics, Quarks are considered a fundamental constituent of matter. When Quarks are combined one forms heavier particles called Hadrons, examples of which are the Proton and Neutron. In total the Standard Model contains six Quarks grouped into three generations. These are the Up and Down Quarks, Charm and Strange Quarks and Bottom and Top Quarks. All Quarks behave as Fermions.
The first generation of Quarks are relatively light and are the elementary particles that compose the components of atomic nuclei. The later generations gradually become more massive with the heaviest particle being the Top Quark. The Top Quark is close to two orders of magnitude larger than the other Quarks making it extremely unstable and practically impossible to isolate. Heavier Quarks are therefore able to decay to lighter Quarks just like in any other theory but this decay must occur through either the Strong, Weak or Electromagnetic Interactions. We thus see Quarks able to mix into other Quarks with a certain degree of probability. In fact the theory of Quark-Confinement forbids the isolation of Quarks and is an important feature of Quantum Chromodynamics; the theory describing the Strong Force.
Each Quark also has a set of characteristics that govern how the Quark can be used to build larger particles and to ensure Quark physics obeys Standard physical laws. These characteristics are know as electric charge, spin, isospin and and colour charge. The latter charge allows Quarks to partake in the Strong Interaction, the force which binds Hadrons in a nucleus. As an example the Up Quark has +2/3 charge while the Down Quark has -1/3 charge. If we combine two Up Quarks and one Down Quark we form the Proton with charge +1. Additionally two Down Quarks and one Up Quark gives a neutrally charged Hadron, the Neutron.
One also finds that additional laws must be introduced to explain Quark physics. These mean introducing additional Quantum numbers for Strangeness (Strange Quarks have value -1) and Topness (Top Quarks have +1) and Bottomness (Bottom Quarks have -1). As with any physical law these quantum values must be conserved in certain interactions.
Antimatter extends into Quark theory with the existance of Anti Quarks. These particles have the same masses, lifetimes and spins as the standard Quarks but have opposing charges (including colour charges). One can therefore pair create Quarks from pure energy as one can pair create an Electron and Positron. The pair production of Quarks is limited by the energy theshold; the more energy available the higher the mass of Quark that can be produced. This is the reason for the late discovery of the Top Quark.