Neutrinos are among the strangest particles in the Standard Model. They carry no electric charge, have an almost immeasurably tiny mass, and interact so weakly with other matter that trillions of them pass through your body every second without you ever noticing.
They come in three types, tied to the electron, muon, and tau, and they are produced in huge numbers by the sun, nuclear reactors, and violent cosmic events like supernovae. Because they interact so rarely, detecting them requires enormous, carefully shielded detectors, often buried deep underground to block out other particles.
One of the biggest surprises in neutrino physics is that they can change from one type to another as they travel, a process called oscillation. That discovery proved neutrinos have mass at all, something the original Standard Model did not predict, and it earned the 2015 Nobel Prize in Physics.