According to the Standard Model of particle physics, muons are a type of unstable fundamental particle that have a charge that is equal to the electron yet have a mass that is 207 times greater (105.6 MeV). Both electrons and muons belong to to the same family of Fermions (i.e fundamental particles) called the Leptons. Because of this, a muon can be thought of as an extremely heavy electron. Muons are denoted by the symbol m.
On earth, muons are created by the absorption of cosmic radiation in the upper atmosphere -- after which they only exist for 2.2 micro-seconds.
Muon neutrino's' have all the characteristics of regular muons, except that they do not have any charge. Muon neutrino's are denoted by n subscript m.
Reference: Serway & Faughn, College Physics, Fourth Edition
(Fort Worth TX: Saunders, 1995) page 841