The neutrino is a type of subatomic particle. As far as we know, neutrinos are fundamental particles: that is, unlike more complicated objects like protons and neutrons, they can’t be broken down into smaller constituents.
Our best current theory of particle physics, the Standard Model, contains two basic types of matter particles: hadrons, which feel the strong force that holds atomic nuclei together, and leptons, which do not feel the strong force. The most familiar lepton is the electron, which has a negative electrical charge and can therefore be detected and manipulated using the electromagnetic force. Electrons are responsible for most of the everyday properties of matter: they determine how different elements react chemically, and flows of electrons make up the electric currents that are so vital to modern technology. In the Standard Model, there are two other charged leptons, the muon (μ) and the tau (τ). Both of these are heavier than the electron and are unstable, decaying within a small fraction of a second, so they have little impact in everyday life, though they are familiar to particle physicists because they are readily created and detected in particle accelerators.
Neutrinos are leptons without electrical charge. This means that they feel only the weak force and gravity. The weak force is aptly named, and also has only a very short range (10-18 m, much less than the diameter of an atomic nucleus), and gravity is unbelievably weak on the atomic scale – every time you pick something up, the electromagnetic force exerted by your muscles easily overcomes the gravitational force of the entire planet Earth – so neutrinos find it extremely difficult to “see” other particles. This makes them very hard to detect – on average, a typical neutrino could pass through a light-year of lead without interacting. They also have extremely low mass, weighing (we believe) less that one-millionth of the mass of an electron, which in turn weighs less than one-thousandth as much as a hydrogen atom. Neutrinos are therefore very elusive creatures, so much so that 25 years elapsed between the first suggestion that they must exist and the first experimental observation.