Neutrino mass hierarchy and lepton flavor mixing

In the standard model of particle physics there are three species of neutrinos whose masses were originally assumed to be zero. But the discovery of solar and atmospheric neutrino oscillations indicates that neutrinos are massive and lepton flavors are mixed. In this brief review we first give an overview of our current knowledge about the neutrino mass spectrum and lepton flavor mixing angles, and then comment on the seesaw mechanisms which allow us to understand the origin of tiny neutrino masses. We pay particular attention to the nearly tri-bi-maximal neutrino mixing pattern and the Friedberg-Lee symmetry to derive it. A relatively promising possibility of detecting hot and warm neutrino dark matter in the Universe will also be discussed.