Fertile Material: Isotopes that are not themselves fissile but can be converted into fissile isotopes through neutron absorption and subsequent nuclear reactions, such as $^{238}U$ or $^{232}Th$.
Fissile Material: Isotopes capable of sustaining a nuclear chain reaction with slow (thermal) neutrons, such as $^{235}U$ or $^{239}Pu$.
Breeder Reactor:A type of nuclear reactor designed to create more fissile material than it consumes by converting fertile isotopes into fissile isotopes.