The neutrino is an elementary particle. It has fractional spin and is therefore a fermion. Its mass is very small compared to most other particles, although recent experiments (see Super-Kamiokande and Sudbury Neutrino Observatory) have shown it to be nonzero. Since it is an electrically neutral lepton, the neutrino interacts neither by way of the strong nor the electromagnetic force, but only through the weak force and gravitation.
Due to the fact that the cross section in weak nuclear interactions is very small, neutrinos can pass through ordinary matter almost unhindered. For typical neutrinos produced in the sun (energy of a few MeV), it would take approximately one light year (~1016m) of lead to block half of them. Detection of neutrinos is therefore very challenging as it has to rely either on large detection volumes or artificially produced neutrino beams of high intensity and energy, since the interaction probability is roughly proportional to the energy of the neutrino.
