The Oscillating Neutrino
Normal matter is made of atoms. Atoms are also composite objects, made up in turn of protons and neutrons (in the nucleus) and the lightweight and familiar electrons. Electrons belong to a class of particles called leptons, the same family to which neutrinos belong. Neutrinos are the very lightweight (originally thought massless) neutral partners of the electrically charged electron and its more exotic cousins the muon and the tau. Physicists use the word flavor to describe these three different kinds of lepton; that is, leptons come in electron flavor (electrons and electron neutrinos), muon flavor (muons and muon neutrinos), and tau flavor (taus and tau neutrinos). Since neutrinos are neutral, they don't feel the electromagnetic force; since they're leptons, they don't feel the strong force. This makes them a useful "weak force only" probe of particle interactions.
It was always thought that neutrinos were massless, but some recent experiments have cast doubt on that assumption. The MiniBooNE experiment will delve more deeply into this crucial question of neutrino mass. So... how do you weigh a neutrino? It turns out that neutrino mass and the curious phenomenon of neutrino oscillations are deeply linked; if neutrinos oscillate, then they must have mass. When neutrinos oscillate, they switch flavors, a phenomenon that violates a Standard Model commandment that a neutrino that starts out as one flavor stays that way forever. (This is sometimes called lepton flavor conservation.) MiniBooNE will start with a beam that's almost 100% muon neutrinos, and then look downstream to see if it can find evidence of other flavors.