The Running Back (RB) is a member of the offense. The RB lines up behind the QB. He receives a handoff, looks for holes in the Defense (“running lanes”), and tries to plow through for as many yards as possible. There are other varieties of this position – Fullbacks, Halfbacks, Tailbacks – but the one you’ll see most often is the running back.
(Famous backs: Jim Brown, Barry Sanders, Walter Payton, Emmitt Smith, Adrian Peterson)