British colonists and the British military in Canada brought rugby to North America. It along with association football became popular in American and Canadian universities. At the time soccer and rugby were not as differentiated as they are now and teams would negotiate the rules before playing a game. The sport of American football evolved from these intercollegiate games.
Meanwhile in England a schism developed between those who favoured strict amateurism and those who felt that players should be compensated for time taken off work to play rugby. In 1895 this resulted in the formation of a break-away sport, rugby league, the rules of the two codes of rugby union and league would themselves diverge.
American football is played on a rectangular field 120 yards (110 meters) long by 53 1/3 yards (49 meters) wide. The longer boundary lines are sidelines, while the shorter boundary lines are end lines. Near each end of the field is a goal line; they are 100 yards apart. A scoring area called an end zone extends 10 yards beyond each goal line to each end line.
Yard lines cross the field every 5 yards, and are numbered from each goal line to the 50-yard line, or midfield. Two rows of lines, known as hash marks, parallel the side lines near the middle of the field. All plays start with the ball on or between the hash marks.
At the back of each end zone are two goal posts (also called uprights) that are 18.5 feet apart (24 feet in high school). The posts are connected by a crossbar 10 feet from the ground.
A try is the rugby equivalent of a touchdown. Unlike American football, rugby league requires the ball to be grounded, in American football it is sufficient for the player carrying the ball to enter the end zone (in-goal area). In American football a touchdown scores 6 points, in rugby league it is worth 4 points.
In all three games, following a try / touchdown, there is the opportunity to score additional points by kicking the ball between the posts and over the bar. In American football this is called an extra point (worth 1 point), in rugby league it is known as a conversion (worth 2 points). There are two key differences between an extra point and a conversion, conversions cannot be charged down like an extra point attempt but they must be taken from the same position as the try was scored. Hence it is important to score under the posts rather than in the corner which makes for a difficult kick.
In American football teams often opt to go for a field goal (worth 3 points) rather than attempt a touchdown. The rugby equivalent is a drop goal worth only one point. The key difference between a field goal and a drop goal is that a field goal attempt is normally kicked with a team-mate holding the ball, whereas in rugby the ball must hit the ground and be kicked from a half-volley.
A similar concept in rugby is the penalty goal. Following the award of the penalty, the attacking team may opt to kick for goal rather than advance the ball by hand or punting. This scores 2 points in league. The penalty goal is similar to a field goal in American football in that the ball is kicked from the ground and may be held by a team-mate, but it cannot be charged. There is no direct equivalent to a penalty goal in American football.
American football has one further method of scoring which does not exist in rugby. If a ball carrier is tackled in their own endzone (in-goal area) with the ball then this results in a safety which scores 2 points. In rugby union this does not score any points but causes the team in possession to kick the ball back to the opposition from under the posts.
In American football, one team retains possession, unless there is a fumble or interception, until they have used up all four downs. Rugby league is very similar with the team in possession having six tackles before having to hand over possession. The key difference is that in American football advancing the ball ten yards earns a new set of downs whereas there is no automatic way of earning a new set of tackles in rugby league.
In both codes, it is normal to advance the ball by carrying it in hand but tactical kicking is an important aspect of play. In American football or rugby league, it is normal to punt on the last down / tackle.
In rugby, forward passes are not allowed and the only form of passing is what would be called a lateral in American football. Only one pass per down may be thrown forwards in American football but any number of laterals may be thrown, in practice laterals are fairly unusual in American football.
In American football team has 11 players on the field at a time. However, teams may substitute for any or all of their players, if time allows, during the break between plays. As a result, players have very specialized roles, and almost all of the 53 players on an NFL team will play in any given game. Thus, teams are divided into three separate units: the offense, the defense and the special teams.
In rugby the same players have to both defend and attack. There are thirteen players in a rugby league team, interchange players can come onto and off the field as long as the maximum number of players is not exceeded.
Many of the positions have similar names but in practice are very different. A fullback in American football is very different from a fullback in rugby, some of the positions are fairly similar; a stand-off carries out a similar role to a quarterback in American football.
Broadly speaking offensive and defensive linemen in American football correspond to to forwards in rugby and other players are somewhat similar to backs.