What is a 'quark'?
(Lansing State Journal, February 12, 1992)
Quarks are believed to be the basic building blocks of protons, neutrons and a wide variety of other, heavier particles, collectively called hadrons. Just as atoms are primarily composed of different combinations of protons, neutrons and electrons, hadrons are believed to be composed of different combinations of quarks. Quarks are usually thought of as elementary particles, i.e. particles that cannot be further broken down into other particles by any known means.
The currently accepted theory of elementary particles predicts that there should be six different kinds of quarks, designated by the letters u (for 'up' quark), d (down), s (strange), c (charmed), b (bottom or beauty), and t (top or truth). The lightest quarks are the 'up' and 'down' quarks, which compose the more common, 'ordinary' particles like protons and neutrons. The 'up' and 'down' quarks are assigned an electric charge or +2/3 of the electronic charge and -1/3 of the electronic charge, respectively. Because no free particles have ever been observed with a charge equal to a fraction of an electronic charge, the quarks must combine in groups of three. For example, a proton is made up of two up quarks and one down quark (uud), giving it a total charge of +1 electronic charges.
The first quark theories were independently proposed by Shoichi Sakata in Japan and Murray Gell-Mann and George Zweig in the United States during the early 1960's. Of all the quarks predicted by the current theory, only the top or truth quark has not yet been observed. Experiments attempting to prove the existence of this quark are currently underway at particle accelerators like the one at Fermilab in Illinois and CERN in Switzerland.
The name 'quark' was originally chosen by Gell-Mann. He took the word from a line in James Joyce's Ulysses: "Three quarks for Muster Mark"
Update (2/23/2010, courtesy of Dr. Ronald Kumon, University of Michigan): The word 'quark' was adopted from James Joyce's "Finnegan's Wake" (not "Ulysses").