term. Refers to an offensive
player who usually lines up behind the quarterback
in the offensive backfield
Running backs usually are handed or tossed the ball on rushing (running) plays. The goal of the running back is to gain yardage and help the offensive team to progress down the football field. Running back (or RBs, for short) also sometimes act as receivers, going downfield and catching passes from the quarterback.
Generally speaking, there are two kinds of running backs. The halfback (also called the tailback) is the main rusher, who gets the bulk of the carries. The term "running back" usually is referring to the halfback. Rarely, the halfback will trick defenses by faking a run, and then passing the ball downfield to a receiver. This is known as a halfback pass.
The fullback is usually bigger, and primarily blocks defensive players, so the halfback can run for more yardage. However, the fullback sometimes rushes the ball himself, especially in situations where the team needs only a short gain (like when near the goal line) and could use the larger fullback to muscle forward for that distance.
There's usually both a halfback and a fullback in the backfield at the start of a play. However, in a single back formation, there'd just be a halfback. An empty backfield might be used in definite passing situations, and have zero running backs in the backfield.
Some of the NFL's most famous running backs include Jim Brown, Gale Sayers, O.J. Simpson, Walter Payton, Earl Campbell, Barry Sanders, Emmitt Smith, Terrell Davis, and Marshall Faulk.