Quarterback

A quarterback (commonly abbreviated to QB) is a position in American and Canadian football. Quarterbacks are members of the offensive team and line up directly behind the offensive line. In modern American football, the quarterback is usually considered the leader of the offensive team, and is often responsible for calling the play in the huddle.

In modern American football, the quarterback is usually the leader of the offense. The quarterback touches the ball on almost every offensive play, and his successes and failures can have a significant impact on the fortunes of his team. Accordingly, the quarterback is among the most glorified and scrutinized positions in team sports. Prior to each play, the quarterback will usually tell the rest of his team which play the team will run. After the team is lined up, the center will pass the ball back to the quarterback (a process called the snap). Usually on a running play, the quarterback will then hand or pitch the ball backwards to a half back or full back. On a passing play, the quarterback is almost always the player responsible for trying to throw the ball downfield to an eligible receiver downfield. Additionally, the quarterback will often run with the ball himself, which could be part of a designed play like the option run or quarterback sneak, or it could be an impromptu effort to avoid being sacked by the defense.

Depending on the offensive scheme by his team, the quarterback's role can vary. In systems like the triple option the quarterback will only pass the ball a few times per a game, if at all, while the pass-heavy spread offense as run by schools like Texas Tech requires quarterbacks to throw the ball in most plays. The passing game is emphasized heavily in the Canadian Football League (CFL), where there are only three downs as opposed to the four downs used in American football, a larger field of play and an extra eligible receiver. Different skillsets are required of the quarterback in each system - quarterbacks that perform well in a pass-heavy spread offensive system, a popular offensive scheme in the NCAA and NFHS, rarely perform well in the National Football League (NFL), as the fundamentals of the pro-style offense used in the NFL are very different from those in the spread system. while quarterbacks in Canadian football need to be able to throw the ball often and accurately. In general, quarterbacks need to have physical skills such as arm strength, mobility, and quick throwing motion, in addition to intangibles such as competitiveness, leadership, intelligence, and downfield vision.

...
Wikipedia