Neutrino is an elementary particle that travels close to the speed of light, lack an electric charge, is able to pass through ordinary matter almost undisturbed and is thus extremely difficult to detect. As of 1999, it is believed that neutrino has a minuscule, but non-zero mass.
Neutrino is created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the sun, in nuclear reactors, or when cosmic rays hit atoms.
The neutrino has half-integer spin and is therefore a fermion. Because 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 gravity.
Because the cross section in weak nuclear interactions is very small, neutrino can pass through matter almost unhindered. For typical neutrinos produced in the sun (with energies of a few MeV), it would take approximately one light year (~10^16m) of lead to block half of them. Detection of neutrino is therefore challenging, requiring large detection volumes or high intensity artificial neutrino beams.
The neutrino is of scientific interest because it can make an exceptional probe for environments that are typically concealed from the standpoint of other observation techniques, such as optical and radio observation.
(Source: http://en.wikipedia.org/wiki/Neutrino)