Leptons are a class of fundamental particle in the Standard Model of particle physics. The class of leptons is comprised of the electron, the muon, the tau, and the three corresponding neutrinos (electron neutrino, muon neutrino, and tau neutrino). All six leptons are spin 1/2 fermions. The quality that distinguishes leptons from other fundamental fermions is that they do not interact via the strong nuclear force, leaving the weak nuclear force, gravity, and, for charged leptons, the electromagnetic force.
Leptons naturally pair up into 'families': the electron and the electron neutrino, the muon and the muon neutrino, and the tau and tau neutrino. Each family contains a charged lepton, with charge -1, and a neutrino which has no charge. These families each have corresponding families of antiparticles, containing positively charged versions of the charged leptons and antineutrinos. All of these particles have a quantity called 'lepton number' that is +1 for leptons and -1 for antileptons. This quantity is generally conserved under all interactions, and non-conserving reactions are strongly suppressed.
Lepton interactions with the electromagnetic force are relatively straightforward, but the weak interactions of leptons have certain subtleties. Most weak interactions between leptons will transform each participant in the interaction into the other member of its family, Because of conservation of charge, this requires that one of the particles be a charged lepton and the other be a neutrino. Thus μ-νe -> e-νμ is allowed but μ-e- -> νμνe is not. Some weak interactions ('neutral current') do not change the identity of the particles involved and thus do not have the same requirement on the identities of the participants. Leptons can also interact with quarks in an analogous fashion.
Weak interactions are also key to the decays of the two unstable leptons, the muon and tau. Rather than interacting with another lepton, in this case the particle spontaneously transforms into a neutrino, emitting a charged W particle which decays to another, lighter charged lepton and a corresponding antineutrino. Muon decay proceeds as μ- -> e-νeνμ (where νe is an electron antineutrino), and tau decay as τ- -> l-νlντ where l represents either an electron or a muon. Electrons and neutrinos are stable and do not decay.
Lepton physics is still a growing and active field. There are ongoing investigations of neutrino mass and neutrino oscillation at neutrino observatories such as SNO and Super Kamiokande. Experimenters are trying to rule out the existence of a fourth family of leptons, and are searching for evidence of a neutral heavy lepton. Finally, the as-yet unproven theory of supersymmetry predicts that leptons have 'partner' bosons called 'sleptons': the selectron, smuon, stau, and sneutrino, and efforts are underway to detect these hypothetical particles.
Sources include my senior undergraduate particle physics course and the (very technical) Particle Data Group website at http://pdg.lbl.gov/
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This writeup is copyright 2004 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.0/ .