Skill positions in gridiron football are the positions that ordinarily handle the ball and are most responsible for scoring points. Offensive players such as quarterbacks, running backs, and wide receivers are typically considered skill positions,    as are tight ends on occasion. 
Skill positions are contrasted with linemen and defensive players, which are generally considered to be positions heavily reliant on power and brute strength.  Skill position players are often physically smaller than linemen, but they must also be faster and have other talents; such as the ability to throw accurately, handle or catch the ball under pressure, avoid tacklers, or read and exploit defensive weaknesses; which are less of a priority for linemen.
The quarterback, colloquially known as the "signal caller", is a position in gridiron football. Quarterbacks are members of the offensive platoon and mostly line up directly behind the offensive line. In modern American football, the quarterback is usually considered the leader of the offense, and is often responsible for calling the play in the huddle. The quarterback also touches the ball on almost every offensive play, and is almost always the offensive player that throws forward passes. When the QB is tackled behind the line of scrimmage, it is called a sack.
A running back (RB) is a member of the offensive backfield in gridiron football. The primary roles of a running back are to receive handoffs from the quarterback to rush the ball, to line up as a receiver to catch the ball, and 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, a wingback or a fullback. A running back will sometimes be called a "feature back" if he is the team's starting running back.
In gridiron football, a lineman is a player who specializes in play at the line of scrimmage. The linemen of the team currently in possession of the ball are the offensive line, while linemen on the opposing team are the defensive line. A number of NFL rules specifically address restrictions and requirements for the offensive line, whose job is to help protect the quarterback from getting sacked for a loss, or worse, fumbling. The defensive line is covered by the same rules that apply to all defensive players. Linemen are usually the largest players on the field in both height and weight, since their positions usually require less running and more strength than skill positions.
Linebacker (LB) is a playing position in gridiron football. Linebackers are members of the defensive team, and line up three to five yards behind the line of scrimmage and the defensive linemen. They are the "middle ground" of defenders, playing closer to the line of scrimmage than the defensive backs (secondary), but farther back than the defensive linemen.
The tight end (TE) is an offensive position in American football, arena football, and Canadian football. It is a hybrid that combines the characteristics and roles of both an offensive lineman and a receiver. Like offensive linemen, they are usually lined up on the offensive line and are large enough to be effective blockers. On the other hand, unlike offensive linemen, they are eligible receivers and potent weapons in team's offensive schemes.
A screen pass is a play in gridiron football consisting of a short pass to a receiver who is protected by a screen of blockers. During a screen pass, a number of things happen concurrently in order to fool the defense into thinking a long pass is being thrown, when in fact the pass is merely a short one, just beyond the defensive linemen. Screens are usually deployed against aggressive defenses that rush the passer. Because screens invite the defense to rush the quarterback, they are designed to leave fewer defensemen behind the rushers to stop the play.
In gridiron football, blitzing is a tactic used by the defense to disrupt pass attempts by the offense. During a blitz, a higher than usual number of defensive players will rush the opposing quarterback, in an attempt either to tackle him or force him to hurry his pass attempt.
This is a glossary of terms used in Canadian football. The Glossary of American football article also covers many terms that are also used in the Canadian version of the game.
Strategy forms a major part of American football. Both teams plan many aspects of their plays (offense) and response to plays (defense), such as what formations they take, who they put on the field, and the roles and instructions each player are given. Throughout a game, each team adapts to the other's apparent strengths and weaknesses, trying various approaches to outmaneuver or overpower their opponent in order to win the game.
A formation in football refers to the position players line up in before the start of a down. There are both offensive and defensive formations and there are many formations in both categories. Sometimes, formations are referred to as packages.
In sports, a starting lineup is an official list of the set of players who will participate in the event when the game begins. The players in the starting lineup are commonly referred to as starters, whereas the others are substitutes or bench players.
In American football, blocking or interference involves legal movements in which one player uses his body to obstruct another player's path. The purpose of blocking is to prevent defensive players from tackling the ball carrier, or to protect a quarterback who is attempting to pass, hand off or run the ball. Offensive linemen and fullbacks tend to do the most blocking, although wide receivers are often asked to help block on running plays and halfbacks may be asked to help block on passing plays, while tight ends perform pass blocking and run blocking if they are not running routes to receive passes. Overall, blocking is a skill that virtually every football player may be required to do at some point, even defensive players in the event of a turnover.
The Tampa 2 is an American football defensive scheme popularized by the Tampa Bay Buccaneers National Football League (NFL) team in the mid-1990s–early 2000s. The Tampa 2 is typically employed out of a 4–3 defensive alignment, which consists of four linemen, three linebackers, two cornerbacks, and two safeties. The defense is similar to a Cover 2 defense, except the middle linebacker drops into a deep middle coverage for a Cover 3 when he reads a pass play.
A halfback (HB) is an offensive position in American football, whose duties involve lining up in the offensive 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, as in the I formation, that player is instead referred to as a tailback.
In American football, the specific role that a player takes on the field is referred to as their "position". Under the modern rules of American football, both teams are allowed 11 players on the field at one time and have "unlimited free substitutions", meaning that they may change any number of players during any "dead ball" situation. This has resulted in the development of three task-specific "platoons" of players within any single team: the offense, the defense, and the so-called 'special teams'. Within these three separate "platoons", various positions exist depending on the jobs that the players are doing.
In American football, a play is a close-to-the-ground plan of action or strategy used to move the ball down the field. A play begins at either the snap from the center or at kickoff. Most commonly, plays occur at the snap during a down. These plays range from basic to very intricate. Football players keep a record of these plays in a playbook.
The prevent defense is a defensive alignment in American football that seeks to prevent the offense from completing a long pass or scoring a touchdown in a single play and seeks to run out the clock, at the expense of allowing short-yardage gains. It is used by a defense that is winning by more than a touchdown, late in the fourth quarter, or in specific situations, such as third-and-very-long if it seems clear that the offense must pass the football to gain long yardage.
The following terms are used in American football, both conventional and indoor. Some of these terms are also in use in Canadian football; for a list of terms unique to that code, see Glossary of Canadian football.
Players in the National Football League wear uniform numbers between 1 and 99, with no two players on a team able to wear the same number outside of the offseason. Rules exist which tie a player's number to a specific range of numbers for their primary position. Additionally, rules exist which limit who may handle the ball on offense: generally players who are designated as offensive lineman, who wear numbers 50–79, are not allowed to handle the ball during a play from scrimmage, though they are allowed to do so if they report to the referee as playing out of position for a tackle-eligible play.