The three leptons, the electron, the muon and the tau particle, are all negatively charged (the charge of their antiparticles is of course positive). The three other leptons, the neutrinos, are among the particles that are most difficult to observe. They have no electrical charge and only a very small mass. They react very seldom with matter and are therefore virtually undetectable.

The three different neutral leptons are the electron neutrino _e, the muon neutrino _µ, and the tau neutrino _t.

There exist in total six leptons and six quarks. The matter particles quarks and leptons are part of a systematic description of the components of matter and the way these interact. It has been shown (see the figure to the left) that there cannot be more than these twelve matter particles grouped into three families.