In the Standard Model quarks are said to be present in six flavors (up, down, charm, strange, top and bottom, often abbreviated by their initial letter as u, d, c, s, t and b), and leptons are said to be present in other six flavors (electron, muon, tau, electron neutrino, muon neutrino and tau neutrino, often abbreviated as e, μ, τ, νe, νμ and ντ).
A fermion of a given flavor is an eigenstate of the weak interaction, that is, it will interact in a definite way with the bosons W+, W- and Z. On the other hand, a fermion of a fixed mass (an eigenstate of the hamiltonian) is normally a superposition of various flavors, and this gives rise to processes that change the flavor. In the case of quarks, it is reflected in the so-called CKM matrix, and the equivalent for neutrinos is the MNS matrix.