Association football, commonly known as football or soccer, is a team sport played between two teams of eleven players, and is widely considered to be the most popular sport in the world. It is a football variant played on a rectangular grass or artificial turf field, with a goal at each of the short ends. The object of the game is to score by manoeuvring the ball into the opposing goal. In general play, the goalkeepers are the only players allowed to use their hands or arms to propel the ball; the rest of the team usually use their feet to kick the ball into position, occasionally using their torso or head to intercept a ball in midair. The team that scores the most goals by the end of the match wins. If the score is tied at the end of the game, either a draw is declared or the game goes into extra time and/or a penalty shootout, depending on the format of the competition.

The modern game was codified in England following the formation of The Football Association, whose 1863 Laws of the Game created the foundations for the way the sport is played today. Football is governed internationally by the Fédération Internationale de Football Association (International Federation of Association Football), commonly known by the acronym FIFA. The most prestigious international football competition is the FIFA World Cup, held every four years. This event, the most widely viewed in the world, boasts an audience twice that of the Summer Olympic Games.

Football is played in accordance with a set of rules known as the Laws of the Game. The game is played using a single round ball, known as the football. Two teams of eleven players each compete to get the ball into the other team's goal (between the posts and under the bar), thereby scoring a goal. The team that has scored more goals at the end of the game is the winner; if both teams have scored an equal number of goals then the game is a draw.

The primary rule is that players (other than goalkeepers) may not deliberately handle the ball with their hands or arms during play (though they do use their hands during a throw-in restart). Although players usually use their feet to move the ball around, they may use any part of their bodies other than their hands or arms.

In typical game play, players attempt to create goal scoring opportunities through individual control of the ball, such as by dribbling, passing the ball to a teammate, and by taking shots at the goal, which is guarded by the opposing goalkeeper. Opposing players may try to regain control of the ball by intercepting a pass or through tackling the opponent in possession of the ball; however, physical contact between opponents is restricted. Football is generally a free-flowing game, with play stopping only when the ball has left the field of play or when play is stopped by the referee. After a stoppage, play recommences with a specified restart.

At a professional level, most matches produce only a few goals. For example, the 2005–06 season of the English Premier League produced an average of 2.48 goals per match. The Laws of the Game do not specify any player positions other than goalkeeper, but a number of specialised roles have evolved. Broadly, these include three main categories: strikers, or forwards, whose main task is to score goals; defenders, who specialise in preventing their opponents from scoring; and midfielders, who dispossess the opposition and keep possession of the ball in order to pass it to the forwards. Players in these positions are referred to as outfield players, in order to discern them from the single goalkeeper. These positions are further subdivided according to the area of the field in which the player spends most time. For example, there are central defenders, and left and right midfielders. The ten outfield players may be arranged in any combination. The number of players in each position determines the style of the team's play; more forwards and fewer defenders creates a more aggressive and offensive-minded game, while the reverse creates a slower, more defensive style of play. While players typically spend most of the game in a specific position, there are few restrictions on player movement, and players can switch positions at any time. The layout of a team's players is known as a formation. Defining the team's formation and tactics is usually the prerogative of the team's manager.

The major international competition in football is the World Cup, organised by FIFA. This competition takes place over a four-year period. More than 190 national teams compete in qualifying tournaments within the scope of continental confederations for a place in the finals. The finals tournament, which is held every four years, involves 32 national teams competing over a four-week period.[41] The 2006 FIFA World Cup took place in Germany; in 2010 it will be held in South Africa.

There has been a football tournament at every Summer Olympic Games since 1900, except at the 1932 games in Los Angeles. Before the inception of the World Cup, the Olympics (especially during the 1920s) had the same status as the World Cup. Originally, the event was for amateurs only, however, since the 1984 Summer Olympics professional players have been permitted, albeit with certain restrictions which prevent countries from fielding their strongest sides. Currently, the Olympic men's tournament is played at Under-23 level. In the past the Olympics have allowed a restricted number of over-age players per team; but that practice will cease in the 2008 Olympics. The Olympic competition is not generally considered to carry the same international significance and prestige as the World Cup. A women's tournament was added in 1996; in contrast to the men's event, full international sides without age restrictions play the women’s Olympic tournament. It thus carries international prestige considered comparable to that of the FIFA Women's World Cup.

After the World Cup, the most important football competitions are the continental championships, which are organised by each continental confederation and contested between national teams. These are the European Championship (UEFA), the Copa América (CONMEBOL), African Cup of Nations (CAF), the Asian Cup (AFC), the CONCACAF Gold Cup (CONCACAF) and the OFC Nations Cup (OFC). The most prestigious competitions in club football are the respective continental championships, which are generally contested between national champions, for example the UEFA Champions League in Europe and the Copa Libertadores de América in South America. The winners of each continental competition contest the FIFA Club World Cup.