  In this report are recalled in a simple form some of the main concepts about
neutrinos, starting from their discovery and classifying them in the Standard
Model of the Microcosm. Then are presented the main natural sources of
neutrinos, emphasizing the enormous number of neutrinos in the Universe. Some
information on neutrinos produced by nuclear reactors and by particle
accelerators are then considered. In the second part is discussed the neutrino
beam sent from CERN to Gran Sasso and the measurement of their velocity
performed by the OPERA experiment, underlying some errors made, their
corrections, and the final results, which represent precision measurements.
