Quarks are one of two groups of fermions. There are six different ‘flavors’ of quarks but only two occur naturally in the modern universe. These are the up and the down quark. The up quark has a charge of +2/3 and is the lightest of all the types of quarks. The other two quarks with a charge of +2/3 are the charm quark and the top quark (the top quark being the heaviest of all the quarks). The down quark, strange quark, and bottom quark all have charges of -2/3 with the down quark being the lightest and the bottom quark being the heaviest of the three.
The charges of the quarks are in terms of the charge of the proton (1.6 * 10^-19 coulombs). Thus the charges of the up, charm, and top quarks are 2/3 * (1.6 * 10^-19 coulombs) or 1.067 * 10^-19 coulombs, and the charges of the down, strange, and bottom quarks are 1/3 * (1.6 * 10^-19 coulombs) or 5.333 * 10^-20 coulombs.
When quarks are combined together baryons and mesons are formed (baryons and mesons are types of hadrons). Baryons consist of three quarks bound together by the strong force. The most common type of baryons are the proton (two up quarks and one down quark) and the neutron (one up quark and two down quarks) which make up the chemical atom. Mesons are made of one quark and one antiquark bound together. Some examples of mesons are the pion (one up quark and one down antiquark), the kaon (one strange quark and one up antiquark), and the B-zero meson (one down quark and one bottom antiquark). There are also three different ‘colors’ quarks which defines how the quark interacts with the strong force and each other. The three different colors of quarks are red, green, and blue. Antiquarks also have their own colors, antired (cyan), antigreen (magenta), and antiblue (yellow).
For every quark there is an antiquark with the same mass and spin, but it has an opposite charge. For example, the up antiquark has the same mass (0.003 GeV/c^2) and spin (1/2) as the up quark but the charge of the up antiquark is -2/3 rather than +2/3. This applies to all of the quarks, so overall there are six flavors and three colors of quarks all with their antiquark counterparts giving a grand total of 36 quarks. Out of all these quarks only two exist in nature. The reason the charm, strange, top, and bottom quarks are not found in nature is because the universe is too cool for them to exist anymore. At the beginning of the universe, right after the big bang, it was hot enough, and therefore enough energy, for the heavier quarks to exist.