A Neutrino is a light electrically-neutral elementary particle, specifically a Fermion, i.e., has half-integer spin, in this way like electrons, quarks and things made of quarks.
Neutrinos are very common (estimated 340 per cubic centimeter throughout the universe, for a total of 1.2×1089) but do not interact very much with Photons or known matter, thus are virtually invisible and pass through solids like the Earth.
The mass of neutrinos is unknown though it is known there are three types, each with its independent mass, some constraints are known regarding the ratio of the masses, and it has been established that the mass is very likely to be less than 0.12 eV/c2. Cosmology-related astronomical observations have as a goal, to determine neutrinos' mass given its total effect on the universe.
Cosmic Background Radiation (CBR)
Core Collapse Supernova
Mikheyev-Smirnov-Wolfenstein Effect (MSW Effect)