The muon is, compared to the electron, a heavy (106 MeV) negatively charged lepton (i.e. a matter particle). It is unstable and therefore it decays into other particles. Its mean lifetime is 2.2 Ás.
Different particles have different capabilities to penetrate through matter, and the muon has a very high penetration capability. Only muons (and neutrinos) have the ability to penetrate first the track detectors and then the calorimeters. The muon detectors are therefore located furthest out in the layer of detectors.
As with hadron calorimeters, muon detectors in CMS consist of layers of iron interleaved with tracking chambers. The muons are detected in the tracking chambers. The muon detectors register charged particles that pass through the hadron calorimeters without creating particle showers.