There are six leptons, three of which have electrical charge and three of which don't. The best known charged lepton is the electron (e). The other two charged leptons are the muon (µ) and the tau (), which are essentially electrons with a lot more mass. The charged leptons are all negative.

The other three leptons are the very elusive neutrinos (). They have no electrical charge and little, if any, mass. There is one type of neutrino for every type of electrically charged lepton (electron, muon, tau).

Leptons are like independent felines because they can exist without the companionship of other particles. Quarks, on the other hand, are only found in groups. Neutrinos are like fleas because they accompany the cats but are very hard to see.

For each of the six leptons there is an antimatter-lepton (antilepton) with equal mass and opposite charge.