The running back is the guy who is typically lined up in the offensive backfield with the quarterback. The primary roles of a running back are to receive handoffs from the quarterback for a rushing play, to catch passes from out of the backfield, and to block. A great running back must be tough, durable, elusive, strong, and an effective blocker. He possesses great vision as well as the ability to make the first tackler miss or can break that first tackle. He is equally effective as a pass receiver out of the back field, as a down field threat, and as a check down in blitzing situations.
Here are some short videos for running backs: