American and Canadian Football both descended from rugby and commenced in Canada as a game played between UK infantrymen garrisoned in Montreal. The squaddies played a sequence of games against scholars at McGill College. McGill played many games against Harvard in 1874 and a custom was born. In spite of their mutual origins, the Canadian and the North American game developed differently and now have significantly different rules and laws.

The largest difference between Canadian and American Football is the dimensions of the field. In Canada, Football fields are 110 yards long and sixty five yards wide. In American Football fields are hundred yards long and 53 and 1/3 yards wide. The goal posts in Canadian Football are placed at the front, instead of the back of the end area which is also deeper in Canadian football than American.

Canadian Football groups have 12 players as against the eleven on American groups. As the same number of players is necessary at the line of scrimmage in both games, this leads to an additional backfield player on Canadian groups. This implies that the classic Canadian offensive setup has 2 slot backs rather than a tight end and on defense, 2 defensive halfbacks and one safety rather than 2 safeties as is characteristic in the American game. Another difference between the two games is the amount of downs. Rather than 4 as in the North American game, Canadian Football has 3. This leads to a more pass and kick orientated game as there are fewer downs available for short-yardage running plays. The kicking rules are also subtly different with the kicker having the ability to recover and advance his very own kick. For that reason, kicking is a far more integral part of Canadian Football than American. The most important difference between the kicking rules in the 2 games is that there's no fair catch rule in Canadian Football.
In American Football, if a kick returner thinks he won't be ready to advance the ball after recovery, he will signal for a fair catch and be immune from contact. In Canadian Football no player on the kicking team excepting the kicker and any players behind him on the field may ever be inside five yards of the ball unless it's been touched by an adversary.
Any kick that goes into the end sector is a live ball, apart from successful field goals, in Canadian Football. There are more minor differences as well including scoring, motion and time rules but they aren't as heavy as the massive differences observed here.