Defender (association football) explained
In association football (soccer), a defender is an outfield player whose primary role is to prevent the opposition from scoring.
There are four types of defender - centre back, sweeper, full back, and wing back.
The job of the centre back, centre half, central defender, or stopper is to stop opposing players, particularly the strikers, from scoring, and to bring the ball out from their penalty area. As their name suggests, they play in a central position.
The position was formerly referred to as centre half, although the emphasis of the centre half was more forward thinking in action. In the early part of the 20th century, when most teams employed the 2-3-5 formation, the two players at the back were called full backs and the row of three players in front of them were called half backs. As formations evolved, the central player in this trio, the centre half, moved into a more defensive position on the field, taking the name of the position with them. The right and left players in the trio were called the right half and left half respectively.
In the modern game, most teams employ two centre backs, stationed in front of the goalkeeper. There are two main defensive strategies used by centre backs: the zonal defence, where each centre back covers a specific area of the pitch, and man-to-man marking, where each centre back has the job of covering a particular opposition player.
The main responsibilities of centre backs include:
- Marking threats in their immediate area (zonal defending) or to mark a particular opposition player (man marking).
- Making tackles on attacking players as the last line of defence against the opposition.
- Intercepting dangerous crosses, shots and through passes that cause an immediate danger to the team.
- Defending opposition corners and to stay in the opposition team's penalty box for corners and set pieces with headers. 
- To form the tactical base of the team. Coaches either choose to field the "line" of defence deep or further upfield. The choice of defensive line has great tactical implications in the professional game. A deep defensive line is considered a more conservative approach; however, it also means that the midfielders require to cover more ground in the game and risks ceding midfield territory to the opposition. A high defensive line allows teams to pin opposition teams into their own territory and apply pressure when chasing for a goal, however, it leaves the attacking team vulnerable to counter attacks by pacy opposition players.
- Maintain the defensive posture. Due to the advent of the offside rule in the modern game, defenders and centre-backs in particular need to ensure that the defensive line is strictly enforced when opposition attacking players are near by. A defender that strays slightly behind the defensive line, for example, can "play" an opposition player onside and inadvertently create a scoring opportunity.
- Playing a simple game. Due to their proximity to goal, centre backs need to avoid over-elaboration and play short and simple passes to their colleagues upfield as well as avoid playing passes square across the defence, where they are vulnerable to interception by an opposition player.
Centre backs require a specialised range of attributes, which include:
- Height, good heading and jumping ability to contest balls in the air.
- Strength, marking and tackling ability to deal with one-on-one threats.
- Bravery and aggression to rush out and deal with any unexpected goal threats.
- Ability to read the game as well as anticipate incoming threats. Centre backs need to balance the need to deal with any imminent threat from the opposition and the need to maintain the defensive posture.
- Concentration. Centre backs need to focus on the task at hand and get interceptions, last-ditch tackles and headers correctly every time. They also need to ensure that they get a sure footing on the ball when clearing it so as to play it away from danger.
The sweeper is a more versatile type of center back that "sweeps up" the ball if the opponent manages to breach the defensive line. His or her position is rather more fluid than other defenders who man-mark their designated opponents. Because of this, the position is sometimes referred to as libero (from the Italian word meaning "free", as used by the sports journalist Gianni Brera). The sweeper's ability to read the game is even more vital than for a centre back. A sweeper is sometimes expected to build counter-attacking moves, and as such requires better ball control and passing ability than a typical centre back. However, sweepers are often merely defensive players. For example, the catenaccio system of play, used in Italian football in the 1960s, employed a purely defensive sweeper who only 'roamed' around the back line.  Franz Beckenbauer is perhaps the greatest ever sweeper to have graced the game. In the 1974 FIFA World Cup final against the Netherlands, he, Paul Breitner and Berti Vogts manmarked Johan Cruyff so well that the Dutch were never able to fully employ their "total football" tactics. Consequently, the Germans won when Müller slotted in the winning goal.
Some sweepers move forward and distribute the ball up-field, while others intercept passes and get the ball off the opposition without needing to hurl themselves into tackles. In modern football, its usage has been fairly restricted, with few clubs in the biggest leagues using the position.
The full backs take up the wide defensive positions, one on each side of the field. Their main task is to prevent opposition players crossing or cutting the ball back into the penalty area. In some defensive systems, full backs man-mark opponents. Most full backs are also expected to provide an attacking dimension by getting upfield along the wings and providing crosses.
In the traditional 2-3-5 team formation, the two players in the final row of defence before the goalkeeper were referred to as full backs. This formation is little used in the modern game, having been replaced largely by the four-man defence, but the term "full back" lives on — the full backs now occupy the wide positions in the defensive line, with the old centre half [back] doubled-up to fill the central defensive position.
The traditional English full back was a large, strong man who would make substantial use of "hacking" - deliberately kicking the shins of opponents, a practice that was accepted as legal in Britain but not in other countries, and caused major controversy as the game became increasingly internationalized from the 1950s on. It is now effectively banned everywhere, and it is this in part that has given rise to a different set of defensive roles.
In the modern game, full backs have taken on a more offensive role than is the case traditionally.  Wingerless formations such as the diamond 4-4-2 formation demand the full back to cover considerable ground up and down the flank. Some of the responsibilities of modern full backs include:
- Provide a physical obstruction to opposition attacking players by shepherding them towards an area where they exert less influence. They may manoeuvre in a fashion that causes the opponent to cut in towards the centre back or defensive midfielder with his weaker foot, where he is likely to be dispossessed. Otherwise, jockeying and smart positioning may simply pin back a winger in an area where he is less likely to exert influence.
- Making off-the-ball runs into spaces down the channels and supplying crosses into the opposing penalty box.
- Throw ins are usually assigned to full backs.
- Marking wingers and other attacking players. Full backs generally do not commit into challenges in their opponents' half. However, they aim to quickly dispossess the attacking players who have already breached the defensive line with a sliding tackle from the side. Markers must however avoid keeping too tight on opponents or risk disrupting the defensive organisation. 
- Maintaining tactical discipline by ensuring other team mates do not over run the defensive line and inadvertently play an opponent onside.
- Providing a passing option down the flank, for instance, by creating opportunities for sequences like one-two passing moves.
- In wingerless formations, full backs need to cover the roles of both wingers and full backs, although defensive work may be shared with one of the central midfielders.
- Additionally, attacking full backs help to pin both opposition full backs and wingers deeper in their own half with aggressive attacking intent. Their presence in attack also forces the opposition to withdraw players from central midfield, which the team can seize to its advantage. 
Due to the physical and technical demands of their playing position, successful full backs need a wide range of attributes, which make them suited for adaptation to other roles on the pitch. Many of the game's utility players, who can play in multiple positions on the pitch, are natural full backs. A rather prominent example is the Inter Milan full back Javier Zanetti, who has played on both flanks as a full back and as a winger, and in central midfield throughout his illustrious career. The more common attributes of full backs, however, include:
- Pace and stamina to handle the demands of covering large distances up and down the flank.
- A healthy work rate and team responsibility.
- Marking and tackling abilities and a sense of anticipation.
- Good off-the-ball ability to create attacking opportunities for his team by running into empty channels.
- Dribbling ability. Many of the game's eminent attacking full backs are excellent dribblers in their own right and occasionally deputise as attacking wingers.
- Player intelligence. As is common for defenders, full backs need to decide during the flow of play whether to stick close to a winger or maintain a suitable distance. Full backs that stay too close to attacking players are vulnerable to being pulled out of position and leaving a gap in the defence. A quick passing movement like a pair of one-two passes will leave the channel behind the defending full-back open. This vulnerability is a reason why wingers considered to be dangerous are double-marked by both the full-back and the winger. This allows the full back to focus on holding his defensive line. 
The wingbacks are a modern variation on the full back with heavier emphasis on attack. The name is a portmanteau of "winger" and "back". They are usually employed in a 3-5-2 formation, and could therefore be considered part of the midfield. But they may also be used in a 5-3-2 formation and therefore have a more defensive role.
In the evolution of the modern game, wingbacks are the combination of wingers and fullbacks. As such it is one of the most physically demanding positions in modern football. Wingbacks are often more adventurous than full backs and are expected to provide width, especially in teams without wingers. A wingback needs to be of exceptional stamina, be able to provide crosses upfield and defend effectively against opponents' attacks down the flanks. A defensive midfielder is usually fielded to cover the advances of wingbacks.
Notes and References
- http://news.bbc.co.uk/sport1/hi/football/rules_and_equipment/4197154.stm BBC SPORT | Football | Laws & Equipment | Positions guide: Central Defender
- http://news.bbc.co.uk/sport1/hi/football/rules_and_equipment/4197172.stm BBC SPORT | Football | Laws & Equipment | Positions guide: Sweeper
- Web site: Football is Coming Home to Die-Hard Translators. Article on the Translation Journal. April 01, 2008. April 14. 2008.
- Web site: Fleet-of-foot full-backs carry key to effective attacking. 2008-12-11. David Pleat explains in a Guardian article how full backs aid football teams when attacking.
- Web site: How Gunners can avoid being pulled apart by Brazilian. 2008-12-11. David Pleat explains the team effort in marking an attacking player stationed in the outside-wing position.
- Web site: How Larsson swung the tie. 2008-12-11. David Pleat explains how the introductions of Barcelona full back Juliano Belletti and striker Henrik Larsson in the Champions League 05/06 final improved Barcelona's presence in wide areas. Belletti eventually scored the winning goal for the final.
- Web site: City countered by visitors' Petrov defence. 2008-12-11. David Pleat discusses the tactical implications of full backs and other defenders marking wingers in a Guardian match analysis.
- http://news.bbc.co.uk/sport1/hi/football/rules_and_equipment/4197076.stm BBC SPORT | Football | Laws & Equipment | Positions guide: Wingback