Football refers to a number of sports that involve, to varying degrees, kicking a ball with the foot to score a goal. The most popular of these sports worldwide is association football, more commonly known as just "football" or "soccer". Unqualified, the word football applies to whichever form of football is the most popular in the regional context in which the word appears, including association football, as well as American football, Australian rules football, Canadian football, Gaelic football, rugby league, rugby union and other related games. These variations of football are known as football codes. Various forms of football can be identified in history, often as popular peasant games. Contemporary codes of football can be traced back to the codification of these games at English public schools in the eighteenth and nineteenth century. The influence and power of the British Empire allowed these rules of football to spread, including to areas of British influence outside of the directly controlled Empire,[4] though by the end of the nineteenth century, distinct regional codes were already developing: Gaelic Football, for example, deliberately incorporated the rules of local traditional football games in order to maintain their heritage. In 1888, The Football League was founded in England, becoming the first of many professional football competitions. In the twentieth century, the various codes of football have become amongst the most popular team sports in the world.
Common Elements
The various codes of football share the following common elements:

    Two teams of usually between 11 and 18 players; some variations that have fewer players (five or more per team) are also popular.
    A clearly defined area in which to play the game.
    Scoring goals or points, by moving the ball to an opposing team's end of the field and either into a goal area, or over a line.
    Goals or points resulting from players putting the ball between two goalposts.
    The goal or line being defended by the opposing team.
    Players being required to move the ball—depending on the code—by kicking, carrying, or hand-passing the ball.
    Players using only their body to move the ball.
In most codes, there are rules restricting the movement of players offside, and players scoring a goal must put the ball either under or over a crossbar between the goalposts. Other features common to several football codes include: points being mostly scored by players carrying the ball across the goal line; and players receiving a free kick after they take a mark or make a fair catch.
Peoples from around the world have played games which involved kicking or carrying a ball, since ancient times. However, most of the modern codes of football have their origins in England.

                                                                                                                                  [Source: Wikipedia]