Quarks are fermions, as they have a half integer spin. There are 6 flavors of quarks: up, down, charm, strange, top and bottom. Quarks are bound by gluons into composite particles called hadrons. Quarks are never observed in isolation, however, only bound together. This is a phenomenon known as color confinement. The theory of this interaction is called quantum chromodynamics. Quarks are the only particles of the Standard Model that interact through all four of the fundamental interactions. Two examples of a hadron are the baryon and the meson.
The baryon is composed of 3 quarks. These are the familiar particles like the proton and neutron that makes up the atomic nucleus. The proton is composed of 2 up quarks and 1 down quark, while the neutron is 1 up quark and 2 down quarks. This is a central idea for the Standard Model.
A meson is composed of a quark-antiquark pair. An antiquark will have a bar over it's symbol.
The quark model was first proposed in 1964 by Murray Gell-Mann and George Zweig.
These are the 3 generations of quarks. Each successive generation is more massive than the previous generation. Through a process known as particle decay, the heavier quarks decay into the up and down quarks. The first generation quarks do not decay and they have the lowest mass. The up and down quarks are the most common and the most stable. The heavier quarks are only produced in high energy collisions
Murray Gell-Mann
George Zweig (originally called quarks "aces")