In contrast to the quarks, any of the six leptons may by found by itself. The electron is the best known lepton. The other three leptons are very elusive particles called neutrinos , which have no electric charge and very little, if any, mass. There is one type of neutrino corresponding to each type of electrically charged lepton. For each of the six leptons there is an antilepton with equal mass and opposite charge.
All forces are due to the underlying interactions of the particles.
Gravity is perhaps the most familiar force to us, but it is not included in the Standard Model because its effects are tiny in particle processes and, furthermore, physicists have not yet figured out how to include it. Atoms combine to form molecules or crystals because of electromagnetic effects due to their charged substructure. Most everyday forces, such as the support of the floor or friction, are due to the electromagnetic forces in matter that resist displacement of atoms or electrons from their equilibrium positions in the material. But we depend on them for the existence of all the stuff from which the world is made, and for the decay processes that make some types of matter unstable. The binding of protons and neutrons to form nuclei is a residual strong interaction effect due to their strongly-interacting quark and gluon constituents.