  With the educated, interested non-specialist as the target audience, we
overview what is known and not known about contemporary neutrino physics.
Theory tells us that neutrinos are the second-most common particle in the
Universe, behind only the quanta of radiation called photons. Almost a trillion
neutrinos per second enter each human eyeball, and yet we do not see them;
these neutrinos, in roughly equal numbers, are emanations from our Sun and
relics of the hot "big bang" era of the early Universe. Much of what we know
about neutrinos, and hope to learn in the future, is derived from a unique
feature of neutrinos -- "oscillation" among neutrino "flavor" types. An initial
neutrino flavor will in general oscillate into another flavor as the neutrino
propagates in space and time. Oscillations are a quantum mechanical phenomenon.
One of the wonders of neutrinos is that their quantum mechanics may be observed
over large distances, even astronomically large. We begin this article with
neutrino phenomenology in terms of masses and mixing angles, including the
matter-antimatter asymmetric "phase" that appears with three neutrino types.
Next we venture into neutrino oscillations. We conclude with a discussion of
model-building for the neutrino masses and mixings. Throughout this article,
attention is devoted to the 2012 result that the "small" neutrino mixing angle
is not so small, after all.
