Halfback (American football)

A halfback (HB) is an offensive position in American football, whose duties involve lining up in the backfield and carrying the ball on most rushing plays, i. e. a running back. When the principal ball carrier lines up deep in the backfield, and especially when that player is placed behind another player (usually a blocking back), as in the I formation, that player is instead referred to as a tailback (see History below).

The halfback position is one of the more glamorous positions on the field, and is commonly viewed as a requirement for a team's success. Sometimes the halfback can catch the ball from the backfield on short passing plays as he is an eligible receiver. Occasionally, they line up as additional wide receivers. When not running or catching the ball, 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.

Before the emergence of the T-formation in the 1940s, all members of the offensive backfield were legitimate threats to run or pass the ball. Most teams used four offensive backs on every play: a quarterback, two halfbacks, and a fullback. The quarterback began each play a quarter of the way back, the halfbacks began each play side by side and halfway back, and the fullback began each play the farthest back.

Historically, from the 1870s through the 1950s, the halfback position was both an offensive and defensive position. Now that most offensive formations have only one or two running backs, the original designations do not mean as much, as the fullback is now usually a lead blocker (technically a halfback), while the halfback or tailback (called such because he stands at the "tail" of the I) lines up behind the fullback. There has also been a shift in most offense’s dependence on halfbacks, as the quarterback is now generally considered the most essential part of a team. However, the average output of the halfback has not changed.

...
Wikipedia