Neutrinos are a type of particle similar to the electron, and belong to the lepton family of fundamental particles. Neutrinos are distinguished by a lack of charge and a mass that's virtually non-existent.
Emitted as a part of the beta decay process that turns neutrons into protons, neutrinos are one of the most abundant of subatomic particles in the Universe, with tens of billions hitting your fingernail every second.
Not that you'd notice - the neutrino is only affected by the pull of gravity and the weak subatomic force, meaning nearly all of them will pass straight through. Their near-zero mass and indifference to other forces lends them the popular name 'ghost particle'.
Flavours of neutrino
Experiments conducted in the 1960s suggested neutrinos came in more than one form.
In addition to the original neutrino that had been identified in association with the emission of electrons, a second variety was associated with another, more recently discovered (and much heavier) member of the lepton family, the muon.
With experimental proof of yet another lepton in 1975 called the tau, suspicions arose that there also had to be a third neutrino.
Sure enough, it's now accepted there are three so-called flavours of neutrino: electron neutrino, muon neutrino and tau neutrino, named after the other three leptons. While they are all distinct, differences in the types of neutrinos emitted by the Sun suggest the same particle can oscillate between different flavours.
Some have even speculated that there is a fourth flavour, one that doesn't even interact through the weak force. This 'sterile' variety is purely hypothetical, and doesn't fit into the Standard Model of physics. Not that it's stopped researchers from looking.