The Fullback (FB) is a member of the offense and is a variation of the running back position. Fullbacks are usually the biggest of all the running backs and split time between running and blocking. When in a formation with multiple running backs, the fullback is the player lined up between the quarterback and the halfback (so he can block for the halfback).
(Famous backs: Jim Brown, Barry Sanders, Walter Payton, Emmitt Smith, Adrian Peterson)