Muons are subatomic charged particles about 200 times as massive as electrons. The primary source of muons on earth is cosmic rays, most of which are hydrogen nuclei (protons). Cosmic-ray protons impact atomic nuclei in the earth's atmosphere, creating pions, which quickly decay into muons. They aren't of great significance to the universe as a whole because of their rapid decay, but they are of historical interest because they were the first subatomic particle discovered that wasn't found in ordinary matter (i.e., a proton, neutron, or electron). In the lab, they serve as sensitive probes to measure magnetic fields because of their unique properties (including the ability to fom short-lived "atoms" by replacing an electron or proton, depending on whether the muon carries a positive or negative charge).
Source(s):
http://cosmic.lbl.gov/SKliewer/Cosmic_Rays/Muons.htm
http://www.isis.rl.ac.uk/aboutIsis/index.htm?content_area=/aboutIsis/whatis...;

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