Roughly speaking, "Frankie Valli & the Four Seasons" is the billing given to the Four Seasons after the group's 1960s heyday, as lead singer Valli became more prominent and personnel changes began to reduce the others to his backup band. In practice, the two billings can be interchangeable; many compilations containing the group's '60s hits as well as Valli solo recordings are credited to Frankie Valli & the Four Seasons as well.
A New Jersey-based vocal quartet that was also nearly a self-contained band (they carried a drummer who was not at first listed as a full-fledged me...