Running back

A running back (RB)/ halfback (HB) is an American and Canadian football position, a member of 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 (in certain contexts also referred to as a tailback) or a fullback. A running back will sometimes be called a "feature back" if he is the team's starting running back.

The halfback (HB) or tailback (TB) position 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 the modern game, an effective halfback must have a blend of both quickness and agility as a runner, as well as sure hands and good vision up-field as a receiver. Quarterbacks depend on halfbacks as a safety valve receiver when primary targets downfield are covered or when they are under pressure. Occasionally, halfbacks 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. As a trick play, running backs are occasionally used to pass the ball on a halfback option play or halfback pass.

The difference between halfback and tailback is the position of the player in the team's offensive formation. In historical formations, the halfback lined up approximately halfway between the line of scrimmage and the fullback (similarly, quarterbacks lined 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), modern offensive formations have positioned the halfback behind the fullback (at the "tail end" of the formation), to take advantage of the fullback's blocking abilities. As a result, some systems or playbooks will call for a tailback as opposed to a halfback.

...
Wikipedia