Neutrinos are (supposed to be) massless particles that are involved in the conservation of momentum. They were 'discovered' by Pauli (I think).There are three flavours of neutrino: electron, muon and tau. Each of these derives from a process involving its name. ie an electron neutrino comes from processes involving electrons (eg beta decay) etc...
The Standard Model says that neutrinos should be massless, and to that end, physicists have measured upper limits for the neutrino mass.
So far, Earth based experiments looking for the neutrino ocsillation haven't seen anything.
Side note: Interestingly, the Isotope science lab at Curtin Uni has done some experiments looking at the mass of a neutrino. They were looking for discreapancies in the double beta decay of Molybdenum. They did n't show that they were massless, but the did give an upper limit. Just goes to show that you don't need bigg instruments to do cosmological experiments. 
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"Once you have pulled the pin from Mr. Grenade, he is no longer your friend."
-Unknown (cited in nene, 2001)
the most interesting result of which is that string theory should unite general relativity and quantum gravity, allowing us to figure out mathematically what happens inside a blackhole or what happened in the first planck-second of the universe 
