Quarks are the building blocks of the protons and neutrons composing the atomic nucleus. Up quarks (charge: 2/3 of the electron's charge) and down quarks (charge: -1/3) come into play: two up quarks and one down quark form a proton (charge: +1), two down quarks and one up quark a neutron (charge: 0). Besides their electric charge, quarks carry a so-called colour charge. The latter does not possess two states (+/-) like the electric charge does, but rather three (red/green/blue). Quarks are however not coloured themselves, the colours are merely a visual representation of the charge state. Besides the up and down quarks, there are four more types of quarks. These are produced in particle collisions, but decay immediately afterwards. They don't play any role in our visible matter, but are interesting study objects for researchers. By the way, quarks never appear as free particles.