What's a Neutrino?
A neutrino is a very small piece of matter. It's so small that is wasn't even discovered until 1956. There are three 'flavors' of neutrinos: electron, muon and tau. Most people are familiar with three particles of matter: electron, neutron and proton. Back in 4th grade science class you learned all about what makes up an atom. There are many more parts than just those three, however.
Protons and neutrons fall into a category of particles called fermionic hadrons, or baryons. Fermions in general are the building blocks of matter. Baryons are made up of quarks, and there are six (6) types of quarks resulting in about one-hundred twenty 120 baryons.
Neutrinos however fall into a category called leptons. Leptons are also fermions, and together with quarks make up matter. The difference between leptons and quarks, is that leptons exist on their own, where quarks combine to form baryons. There are 6 types of leptons: electron, electron neutrino, muon, muon neutrino, tau and tau neutrino. For each of these, the neutrino brand carries a neutral charge, while their counterparts all have a negative charge. Note: There is no correlation between the particles in the same rows, it's just how the table lines up.
Now there is another class of particles called Bosons. They are also made up of quarks, like fermions. The property that separates bosons from fermions is called spin. If it has an integer spin, it is a boson. If it has a spin +- 1/2, +- 3/2 then it is a fermion. Another class of particles is known as mesons, which are also bosons. One example of a meson that will come up later is the pion. While Fermions are the matter paticles, it would be convenient to think of bosons as the mediators of force: where the gluons mediate strong force, photons W+, W- and Z particles mediate electroweak force, and the as yet unobserved graviton mediates the gravitational force. Mesons are also bosons since they too are made of quarks, with integer spins.
This is a very basic description of what is known as the Standard Model of Fundamental Particles. But it is also important to know that there are several forces that act upon particles: gravitational, weak, electromagnetic and strong. Each type of particle interacts differently to each force. For example, every particle with mass interacts gravitationally, but only electrically charged particles interact electromagnetically. Quarks and gluons interact via the strong force, and quarks and leptons interact weakly.
For a more comprehensive diagram look at the Standard Model of Fundamental Particles and Interactions
Now if we think back to the properties of the neutrino, it is a lepton with a neutral electric charge. Since leptons are their own constituents, not made up of quarks or gluons, that implies that there is only two ways a neutrino will interact: gravitationally and weakly (however neutrinos generally have such small mass, that gravitational effects are negligible). This is very important to neutrino astrophysics. As other particles traverse galactic and exra-galactic distances, they can become deflected, scattered, or even stopped altogether by matter, gravitational and magnetic fields. Neutrinos can pass through all of these uninhibited, which makes them excellent sources of information from the far reaches of the galaxy.