What are "quarks" and how do they fit into the modern atomic theory?

Answer 1

Lets see if I remember my physics classes correctly: A quark is a theoretical "missing link" sub-atomic particle. Before they were "invented", astronomic and atomic theory did not fit with observed facts. They are (theoretically) one of the two basic constituents of matter (the other is the lepton) and form part of the basis of modern Quantum Theory.

That's the way I remember it, the link I've given should have more.
Hope that helps

Answer 2

I'm not an expert, but quarks are a theoretical particle that things like protons and neutrons are made of. They were supposed to be the smalles particle, but this title has now been grabbed by string theory. Quarks have different "flavors", the same way electrons are deemed to have different spins.

If I recall, Dr. Gell-Mann was the quarks guru. Try putting his name into google.