As the name suggests, SET involves the transfer of a single electron from one species to another. The species which donates the electron is oxidised an the species which receives the electron is reduced. If a neutral and spin paired molecular species receives an electron by SET (from a metal, electrode etc.) it becomes a radical anion. For more information look in the Chemogenesis
webbook sections on redox
chemistry and substitution,
transfer, abstraction, displacement (STAD).