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The neutrino is an elementary particle. It has spin 1/2 and so it is a fermion. Its mass is very small, although recent experiments have shown it to be above zero. It feels neither the strong nor the electromagnetic force, so it only interacts through the weak force and gravitation.

Because the neutrino only interacts weakly, when moving through ordinary matter its chance of interacting with it is very small. It would take a light year (10 Pm) of lead to block half the neutrinos flowing through it. Neutrino detectors therefore typically contain hundreds of tons of a material constructed so that a few atoms per day would interact with the incoming neutrinos.