Edge rusher (alternately edge defender or simply edge) is a term designating a position in gridiron football. Some consider edge rusher to be its own position entirely. 
Players considered to be edge rushers are usually 4–3 defensive ends or 3–4 outside linebackers. Note that 3–4 outside linebackers often act as an extension of the defensive line, in that they will attack the offensive tackles or blocking tight ends on the majority of their snaps under a majority of 3–4 schemes, though it is not uncommon to see them drop back and play a more traditional 4–3 linebacker role as well. 
One reason the word "edge" is used in the term "edge rusher" is that edge often refers to the area outside of offensive tackles, but within a couple of yards of the line of scrimmage. While other positions will rarely be referred to as edge rushers, other than 4–3 defensive ends and 3–4 outside linebackers, it is common for defensive backs, traditional linebackers, and even defensive tackles to occasionally play the position on a play-to-play basis. 
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.
A linebacker is a playing position in gridiron football. Linebackers are members of the defensive team, and line up approximately three to five yards behind the line of scrimmage and behind the defensive linemen. They represent the "middle ground" of defenders, playing closer to the line of scrimmage than do the defensive backs, but farther back than do the defensive linemen.
The tight end (TE) is a position in American football, arena football, and Canadian football, on the offense. The tight end is often a hybrid position with the characteristics and roles of both an offensive lineman and a wide 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 adept enough to warrant a defense's attention when running pass patterns.
In gridiron football, a sack occurs when the quarterback is tackled behind the line of scrimmage before he can throw a forward pass, when the quarterback is tackled behind the line of scrimmage in the "pocket" and his intent is unclear, or when a passer runs out of bounds behind the line of scrimmage due to defensive pressure. This often occurs if the opposing team's defensive line, linebackers or defensive backs are able to apply pass pressure to quickly get past blocking players of the offensive team, or if the quarterback is unable to find a back to hand the ball off to or an available eligible receiver to catch the ball, allowing the defense a longer opportunity to tackle the quarterback.
Defensive end (DE) is a defensive position in the sport of gridiron football.
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.
The 46 defense is an American football defensive formation, an eight men in the box defense, with six players along the line of scrimmage. There are two players at linebacker depth playing linebacker technique, and then three defensive backs. The 46 defense was originally developed and popularized with the Chicago Bears by their defensive coordinator Buddy Ryan, who later became head coach of the Philadelphia Eagles and Arizona Cardinals.
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.
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.
Tackle is a playing position in gridiron football. Historically, in the one-platoon system prevalent in the late nineteenth and early twentieth centuries, a tackle played on both offense and defense. In the modern system of specialized units, offensive tackle and defensive tackle are separate positions, and the stand-alone term "tackle" refers to the offensive tackle position only. The offensive tackle is a position on the offensive line, left and right. Like other offensive linemen, their job is to block: to physically keep defenders away from the offensive player who has the football and enable him to advance the football and eventually score a touchdown. The term "tackle" is a vestige of an earlier era of football in which the same players played both offense and defense.
A fullback (FB) is a position in the offensive backfield in gridiron football, and is one of the two running back positions along with the halfback. Fullbacks are typically larger than halfbacks and in most offensive schemes the fullback's duties are split among power running, pass catching, and blocking for both the quarterback and the other running back.
In American football, the 3–4 defense is a common defensive alignment consisting of three down linemen and four linebackers. It is a called a "base defense" because it is the default defensive alignment used on "base downs". However, defenses will readily switch to other defensive alignments as circumstances change. Alternatively, some defenses use a 4–3 defense.
In American football, a 4–3 defense is a defensive alignment consisting of four down linemen and three linebackers. It is called a "base defense" because it is the default defensive alignment used on "base downs". However, defenses will readily switch to other defensive alignments as circumstances change. Alternatively, some defenses use a 3–4 defense.
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.
A defensive tackle (DT) is a position in American football that will typically line up on the line of scrimmage, opposite one of the offensive guards, however he may also line up opposite one of the tackles. Defensive tackles are typically the largest and strongest of the defensive players. Depending on a team's individual defensive scheme, a defensive tackle may be called upon to fill several different roles. These roles may include merely holding the point of attack by refusing to be moved, or penetrating a certain gap between offensive linemen to break up a play in the opponent's backfield. If a defensive tackle reads a pass play, his primary responsibility is to pursue the quarterback, or simply knock the pass down at the line if it is within arm's reach. Other responsibilities of the defensive tackle may be to pursue the screen pass or drop into coverage in a zone blitz scheme. In a traditional 4–3 defense, there is no nose tackle. Instead there is a left and right defensive tackle. Some teams, especially in the National Football League (NFL), have a nose tackle in this scheme, but most of them do not.