Leptons are any member of a class of fermions that respond only to electromagnetic, weak, and gravitational forces and do not take part in strong interactions. Like all fermions, leptons have a half-integral spin. Leptons obey the Pauli exclusion principle, which prohibits any two identical fermions in a given population from occupying the same quantum state.
Leptons can either carry one unit of electric charge or be neutral.
The charged leptons are the electrons, muons, and taus. Each of these types has a negative charge and a distinct mass. Electrons, the lightest leptons, have a mass only 0.0005 that of a proton. Muons are heavier, having more than 200 times as much mass as electrons. Taus, in turn, are approximately 3,700 times more massive than electrons.