What's a quark? The Concise Oxford Dictionary defines it thus:
Any of a group of sub-atomic particles carrying a fractional electric charge, postulated as building blocks of the hadrons invented by the American physicist Murray Gell-Mann and associated with the line 'Three quarks for Muster Mark' in James Joyce's 'Finnegans Wake'
Any clearer? Well, quarks are small. They're about as small as space is big. If you think it's a bummer to drop your laundromat token into the gravel, try recovering one of these from a tangled crystal mesh on a windy day.
There are six kinds of quark: Up, Down, Top, Bottom, Charm and Strange1. Quarks never occur singularly, but in groups of two or three. Three quarks are called a baryon. Protons and neutrons are baryons. Protons consist of two up quarks and a down quark, and neutrons consist of two down quarks and an up quark. Two quarks are called a meson. Mesons are made up of a quark and an antiquark2, and as such are extremely unstable.
Quarks are believed to be the smallest particles in the universe. However, the same was also once believed of atoms, and then of neutrons, protons, and electrons. So keep your eyes peeled for news; any day now a still-smaller piece of matter may be discovered.