three neutrino flavors
Just as the charged leptons come in three versions — the electron, the muon, and the tau — the neutrinos come in three matching versions too. These three types are called the three flavors: the electron neutrino, the muon neutrino, and the tau neutrino. Flavor is just physicists' playful word for kind or type; it has nothing to do with taste.
What makes a neutrino belong to one flavor rather than another is the company it keeps when it is born or caught. An electron neutrino is the one that gets created alongside an electron (or that turns into an electron when it interacts); a muon neutrino partners the muon; a tau neutrino partners the tau. So if a neutrino slams into matter and produces a muon, you say it was a muon neutrino. The flavor is defined by the charged lepton it pairs with through the weak force.
The number of flavors is not arbitrary. Experiments at the LEP collider in the 1990s measured how often the Z boson decays into unseen neutrinos and pinned the count of light, ordinary neutrino types at almost exactly three. That clean result — three flavors, no more, no fewer — is one of the firm pillars of the Standard Model. The twist, discovered later, is that these flavors are not fixed labels: a neutrino can change from one flavor to another as it flies, a phenomenon called oscillation.
A neutrino beam aimed at a detector might enter as muon neutrinos, but the detector occasionally records an electron or a tau coming out of a collision — proof that some of those muon neutrinos changed flavor on the way.
A neutrino's flavor is revealed by which charged lepton it makes.
The LEP result of three flavors counts only neutrinos light enough and ordinary enough to come from a Z decay; it does not rule out hypothetical extra 'sterile' neutrinos that ignore the weak force entirely.