A lepton is an subatomic elementary particle with Spin 1/2 that does not interact strongly. They are all fermions. We know three kinds, called generations, of leptons: the electron, the muon and the tauon. Each is associated with a neutrino: electronic neutrino, muonic neutrino and tau neutrino. In total, these six flavors comprise all known leptons in the Standard Model.
Leptons are characterized by a nonzero lepton number. Each flavor has its own quantum number.