The nucleus of an atom is made of protons and neutrons. But what are protons and neutrons made out of? Small, unappreciated particles called quarks.
There are six kinds of quarks, named up, down, charm, strange, top, and bottom.
Only up and down quarks make up the normal matter that you run into every day. The other four turn out to be not very practical, and are rarely observed in the Universe. This is almost exactly like how some boats are made of wood, and other boats are made out of metal, but you don't often witness seaworthy vessels made out of cellophane, feta cheese, Anna Nicole Smith, or Buddhism.
The top quark is 40 times heavier than the next biggest quark, and is as heavy as an entire gold atom. It doesn't normally exist in the Universe. However, top quarks can be created by high energy collisions inside particle accelerators, where they are briefly impressive, but then become wildly unstable and disappear. In this respect, top quarks are the Crispin Glover of subatomic particles.