Neutrinos are elementary particles which experience only the weak and gravitational interactions and so are very difficult to detect. Neutrinos play an important role in the evolution of the universe, in astrophysical phenomena and in particle physics phenomenology. In addition, neutrinos show the interesting phenomenon of flavor oscillation which has been discovered by the Super-Kamiokande experiment in 1998. The flavor oscillation implies that the neutrinos have non-zero masses, although very small. There are several open questions about the properties of neutrinos: whether or not the CP symmetry is violated by neutrinos? what are the absolute values of the masses? are they Dirac or Majorana particles? do they have some non-standard interactions? Also, not having electric charge and interacting very weakly, neutrinos are one of the best messengers in our universe. They can provide unprecedented information about our galaxy, astrophysical events and the early universe. The recent detection of the extra-galactic neutrinos by the IceCube experiment marked the beginning of the neutrino astronomy, an exciting area with several open questions.