A quark is an elementary particle and a fundamental constituent of matter. Quarks only exist in groups with other quarks and are never found alone. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Quarks are strongly interacting particles and existence of isolated quark is not discovered so far.
There are six types of quarks. They are: up, down, charm, strange, top and bottom.
Also, for each of these quarks, there is a corresponding anti-quark. They are: anti-up, anti-down, anti-charm, anti-top and anti-bottom.
Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively.
The properties of different types of quarks and anti quarks are given in the table below: