Quark

A quark is a generic type of physical particle that interacts via all four fundamental forces and that forms one of the two basic constituents of matter, the other being the lepton. Various flavors of quarks combine in specific ways to form protons and neutrons, in each case taking exactly three quarks to make the composite particle in question.

There are six different types of quarks, usually known as flavors: up, down, charm, strange, top, and bottom. The charm, strange, top, and bottom varieties are highly unstable, and are believed to have decayed within a fraction of a second after the Big Bang – though they can be briefly recreated and studied by scientists. However, the "up" and "down" varieties are abundant and are distinguished by (amongst other things) their electric charge.

It is this which makes the difference when quarks clump together to form protons or neutrons: a proton is made up of two "up quarks" and one "down quark", yielding a net charge of +1, while a neutron contains one "up quark" and two "down quarks", yielding a net charge of 0.