Lepton

A lepton is one type of the fundamental building blocks of matter.

As far as early 21st century scientists know, leptons, together with quarks, are truly fundamental and cannot be broken down further. Basically leptons are electrons, muons and tau particles, along with their complementary antiparticles and neutrinos. Leptons are much lighter than baryons (protons, neutrons and associated antiparticles). Leptons are fermions and have a quantum spin of ½, and each has neutrinos associated with it. Leptons appear point-like without internal structure.

Flavour
They've got exciting flavours, though we can't actually taste them. They'd probably be tasty though. Scientists know different leptons have different properties and for want of a better word they call them flavours  (with a suplerfluous "u" to make them sound fancier).

Cold fusion
Muons are sometimes put forth as a way of performing cold fusion; they can indeed effectively catalyze fusion by displacing an electron in a molecule of H2 and sucking the nuclei close enough together to fuse. Unfortunately, they have a half-life of only about 2.2 microseconds, and additionally have some chance of "sticking" to the reaction products instead of continuing to catalyze more fusion. The total energy yield from the number of reactions a single muon could catalyze is lower than the energy needed to create a muon by presently known methods, and so muon-catalyzed fusion isn't actually useful.

Magnetic moment
The muon is known to have a magnetic moment differing slightly from that predicted by the due to various quantum effects. Measuring just how different can serve as a useful test for various theories; for example, the Standard Model's prediction is about 3.4σ off the observed value, suggesting that something beyond the Standard Model is going on… probably.