  In addition to three active neutrinos, one or more light sterile neutrinos
have been conjectured to account for the LSND, MiniBooNE and reactor
antineutrino anomalies (at the sub-eV mass scale) or for warm dark matter in
the Universe (at the keV mass scale). Heavy Majorana neutrinos at or above the
TeV scale have also been assumed in some seesaw models. Such hypothetical
particles can weakly mix with active neutrinos, and thus their existence can be
detected at low energies. In the (3+3) scenario with three sterile neutrinos we
present a full parametrization of the 6 X 6 flavor mixing matrix in terms of
fifteen rotation angles and fifteen phase angles. We show that this standard
parametrization allows us to clearly describe the salient features of some
problems in neutrino phenomenology, such as (a) possible contributions of light
sterile neutrinos to the tritium beta decay and neutrinoless double-beta decay;
(b) leptonic CP violation and deformed unitarity triangles of the 3 X 3 flavor
mixing matrix of three active neutrinos; (c) a reconstruction of the 6 X 6
neutrino mass matrix in the type-(I+II) seesaw mechanism; and (d) flavored and
unflavored leptogenesis scenarios in the type-I seesaw mechanism with three
heavy Majorana neutrinos.
