What is a Neutrino?
According to Frederick Reines, one of the scientists for the neutrino discovery and and Nobel Prize winner:
"... the most tiny quantity of reality ever imagined by a human being".
Neutrino – means little, neutral one, since it carries no charge, just like neutron and is extremely light and therefore small.
Physicists recently found out that they have mass, but it is so small that we do not know it – all there is known is that it is less than 2 millionth parts of the electron mass. Mass is just one of the unknowns about neutrinos that physicists are looking for. Finding these unknowns is made more difficult because the tiny neutrino rarely interacts with other matter.
Neutrinos are very different than the electrons and protons and neutrons which most are familiar with. They are indivisible leptons, like the electron, but carry no electrical charge, like the neutron. Physicists aren't sure whether or not they have mass, but if they do, it must be very small, even smaller than the electron. Mass is just one of the unknowns about neutrinos that physicists are looking for; finding these unknowns is made more difficult because the tiny neutrino rarely interacts with other matter.
More formally – neutrinos are elementary particles that form 3 out of 12 the most fundamental building blocks of nature that universe as we know it is made of.