According to the Standard Model of particle physics, a muon (also known as a mu meson) is a collective name for two semistable fundamental particles with positive and negative charge. Muons have a mass that is 207 times greater than the electron (105.6 MeV) and a spin of 1/2. Both electrons and muons belong to the same family of fermions (i.e., fundamental particles) called the leptons. Because of this, a negatively-charged muon can be thought of as an extremely heavy electron. Muons are denoted by μ- and μ+ depending on their charge.
On earth, muons are created when a charged pion decays. The pions are created in an upper atmosphere by cosmic radiation and have a very short decay time--a few nanoseconds. The muons created when the pion decays are also short-lived: their decay time is 2.2 microseconds. However, the muons have high energies, so the time dilation effects of special relativity make them easily detectable at the earth's surface.
As with the case of electrons there is a muon neutrino which is associated with the muon. Muon neutrinos are denoted by νμ.
Positive muons can form a particle called muonium, or μ+e–. Due to the mass difference between the muon and the electron, muonium is more similar to atomic hydrogen than positronium. Muonium has been used to produce muon-catalyzed fusion in which muons shield the positive charge of the nuclei so that the nuclei can fuse.
Reference: Serway & Faughn, College Physics, Fourth Edition (Fort Worth TX: Saunders, 1995) page 841