Lepton number is a conserved quantity in particle physics that helps classify particles known as leptons. Each lepton, such as electrons and neutrinos, has a lepton number of +1, while their antiparticles have a lepton number of -1. Other particles, like quarks, have a lepton number of 0. In any physical process, the total lepton number before and after must remain the same. This conservation law is important in understanding interactions in particle physics, such as those occurring in beta decay or during high-energy collisions in particle accelerators.