American and Canadian football both descended from rugby and began in Canada as a sport played between British soldiers garrisoned in Montreal. The soldiers played a series of games against scholars at McGill University. McGill played several games against Harvard in 1874 and a tradition was born. Despite their mutual origins, the Canadian and the American sport developed differently and now have substantially varied rules and regulations.
The finest difference between Canadian and American football is the dimensions of the playing field. In Canada, football fields are 110 yards long and 65 yards wide. In America football fields are 100 yards long and fifty three and 0.33 yards wide. The goal posts in Canadian football are placed at the doorway, rather than the back of the end zone which is also deeper in Canadian football than American.
Canadian football teams have twelve players as opposed to the eleven on American teams. Because a comparable range of players is required at the line of scrimmage in both games, this leads to an additional backfield player on Canadian teams. This means that the usual Canadian offensive setup has two slot backs instead of a tight end and on defense, two defensive halfbacks and one security instead of two safeties as is usual in the American sport.
Another difference between the two games is the range of downs. Instead of four as in the American sport, Canadian football has three. This leads to a more pass and kick oriented sport since there are fewer downs to be had for brief-yardage working plays. The kicking rules are also a contact varied with the kicker being able to upgrade and advance his own kick. For this reason, kicking is a most more integral detail of Canadian football than American.
The finest difference between the kicking rules in the two games is that there isn't a fair seize rule in Canadian football. In American football, if a kick returner thinks he's going to not be able to advance the ball after recovery, he can signal for a good seize and be immune from contact. In Canadian football no player on the kicking team excluding the kicker and any players behind him on the field could ever be within 5 yards of the ball unless it has been touched by an opponent. Also, in Canadian football any kick that goes into the end zone is a reside ball, excluding for successful field goals.
There are other minor differences as properly including scoring, motion and time rules however they can be not as significant as the major differences noted here.