According to the Backstage directory site, they are one and the same.
But having asked an online mate who's still in Whollyodd (via Georgia) and does casting, amongst other things...
...it seems in film and TV, they're now more commonly referred to as background actors / featured background actors - i.e. you take a scheduled action or speak (even if it's not a scheduled line, for example background babble that has to be on a convincing theme), which is also what they're referred to as by SAG-AFTRA's collective contracts. Extra is now more a stage and musicals thing. Though, in practice, he uses the term interchangeably.