A running back (RB) is the position of a player on an American or Canadian football team who usually lines up in the offensive backfield. 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.
There are usually one or two running backs on the field for a given play, depending on the offensive formation. A running back may be a halfback (HB for short, in certain conexts also referred to as a tailback) or a fullback (FB). While a fullback is technically a running back, in modern parlance the terms "running back" and "RB" are frequently used to refer only to halfbacks/tailbacks.
The halfback or tailback position is one of the more glamorous positions on the field, as it is often integral in both the passing and running attack. A well-rounded halfback is commonly viewed as a requirement for a team's success. He is responsible for carrying the ball on the majority of running plays, and may frequently be used as a receiver on short passing plays. In today's game, an effective halfback must have a superior blend of both quickness and agility as a runner, as well as sure hands and shrewd vision upfield as a receiver. More and more quarterbacks depend on halfbacks as a dump-off receiver when primary targets downfield are covered. Occasionally, they line up as additional wide receivers. When not serving either of these functions, the primary responsibility of a halfback is to aid the offensive linemen in blocking, either to protect the quarterback or another player carrying the football. On some rare occasions, running backs are used to pass the ball on a halfback option play or halfback pass.
No position in American football can perform his duties successfully without the help of other players. Like the wide receiver, who generally cannot make big plays without the quarterback passing to him (with the exception of the end-around or a reverse), the running back nearly always needs good blocking from the offensive line to successfully gain yardage. Also, a running back will generally have more rushing attempts than a receiver will have receptions, which is mainly because a receiver will average 10 to per reception, while running backs will average on a good run.
Running backs tend to benefit greatly from the blocking of the offensive lineman, so much so that a below-average running back can perform superbly with a deft group of blockers.
Note that the difference between halfback and tailback is simply the position of the player in the team's offensive formation. The halfback lines up approximately half-way between the line of scrimmage and the fullback (similarly, quarterbacks line up a quarter of the distance between the line of scrimmage and the fullback). Because the halfback is usually the team's main ball carrier (while the fullback is primarily a blocker), many coaches see fit to position the halfback behind the fullback (at the "tail end" of the formation). In this case, the halfback becomes a tailback.
In high school football, where the offenses are simpler and player sizes vary greatly, fullbacks are still frequently used as ball carriers. In many high school and in some college football scheme known as the "Triple Option," the fullback is considered a primary ballcarrier for this offense. The fullback in this particular offense plays a unique role by establishing an inside running threat during the a triple option. Triple Option scheme fullbacks require excellent ballcarrying skills and employ a tenacious running style, but often lack the receiving skills of their counterparts in other schemes. College teams such as Navy and Air Force have employed the triple option scheme with success.
While in years past the fullback lined up on the field for almost every offensive play, teams often opt to replace the fullback with an additional wide receiver or a tight end in modern football. Fullbacks in the National Football League rarely get to carry or catch the ball since they are used almost exclusively as blockers. Their talent and value to a team is usually judged by the success of the team's halfback or by how many times the team's quarterback is sacked, similar to the way offensive linemen are judged. Fullbacks are also still used occasionally as rushers on plays when a short gain is needed for a first-down, as they are large and powerful and therefore good for breaking through the line for a short distance.
At the other extreme are "power backs": Bigger, stronger players who can break through tackles using brute strength and raw power. They are usually (but not always) slower runners compared to other backs, and typically run straight ahead (or "North-and-South" in football terminology) rather than dodging to the outside edges of the playing field (i.e. running "East-and-West") like shorter, quicker, lighter backs will often do.
Several successful running backs fall somewhere between the extremes of "scat back" and "power back", combining speed and agility with power and strength.
Some teams have a "third down back", who is skilled at catching passes or better at pass blocking and "picking up the blitz," and thus is often put in the game on third down and long. He can also be used to fool the defense by making them think he is being put into the game for a pass play, when the play is actually a run.