In astronomy, what is a quark?
A quark is more of a physics concept as opposed to an astronomy concept, but it is part of a atom.
Atoms are made of electron, protons and neutrons for the most part, these are in turn made up of quarks. These particles come in several forms known as rotations. For a detailed, yet simplified explanation, look into "The Universe in a Nutshell," by Steven Hawking.