Neutrino is a fundamental particle, emitted in various nuclear and particle physics processes. It is the most elusive particle, known for scientists and this property makes it to be almost invisible. We briefly mention history of neutrino and its physical properties. We concentrate especially on the problem of its mass. Neutrinos had been taken as massless particles for a long time. Many recent experiments find so called neutrinos oscillations. The observed oscillations prove that the mass of neutrinos is not zero. We discuss these experiments and their results (especially from Japan Super-Kamiokande and KamLAND). Oscillations can solve problems in other fields of physics. We briefly deal with long standing solar neutrino problem as well as with the relations of neutrinos and dark matter.