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 20 yards (18 m) per reception, while running backs will average 5 yards (4.6 m) 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.