Comparison of American football and rugby union
A rugby union game is divided into two halves of 40 minutes (or shorter for lower-grade games, and breaks given halfway through each half if playing conditions are considered to be extreme) separated by a half time period of up to 15 minutes in an international match. Most notably, a rugby union game will continue after the scheduled end of a half (half-time or full-time) until the ball becomes dead - any occurrence that would have play restart with a scrum or lineout, or when a team scores. This has led to some "nail-biting" finishes where teams losing by only a small margin work their way towards scoring, and games can go on several minutes over time. Something similar can occur at the end of either half in American football, though usually for only a few seconds of additional time, because a play that begins before the half expires continues until the ball is dead regardless of whether there is any time left on the clock. In both sports, the clock is also stopped during substitutions and for injuries, so the referee does not need to add stoppage time as is done in soccer/Association football. American football games are made up of four quarters of 15 minutes each (less at the high school and youth levels), but the clock stops and starts according to specific rules, so that a 15-minute quarter lasts much longer.
Comparison of American football and rugby league
Generally, American football games last much longer than 80-minute rugby league matches. Because the field is reset after each tackle in American football, it is much slower-paced than the more hectic rugby league, in which play stops for only as long as it takes the tackled player to get back to his feet and return the ball to play. Another major difference is that only the player with possession of the ball may be interfered with in rugby league; defending players interfering with any other attacking player (and vice versa) will incur a penalty. Passing in the two sports also differs: while backward passing is common to both sports, in American football players are allowed to throw the ball forward in certain situations whereas in rugby league forward passing is always illegal. rugby league footballers have to kick the ball or run it forward to advance it downfield. To score a try or touchdown in rugby league, the player has to push the ball directly onto the ground once in the try zone, whereas in American football the player has to simply cross the line. American football also requires its players to use a large amount of protective equipment, including helmets, gloves and padding around the body, whereas protective clothing is much more minimal in rugby league, usually amounting to light padding (if any) and soft head gear (hard helmets being illegal). 
The rugby pitch and American football field are both similar in size with a pitch being 100 meters while a football field is 100 yards.