The major differences between American and Canadian (ha! go ahead, everybody, fight about that terminology!) are: (1) the dimensions of the playing field, the Canadian field being larger; (2) the number of players, 11 in American, 12 in Canadian; (3) three downs to advance 10 yards in Canadian, but four downs in American; (4) in Canadian, a team can score one point for a rouge, awarded to a kicking or punting team if an opposing player is trapped in the end zone on a return play.

For more details, see http://www.cuug.ab.ca/~leblancj/stampeders/compare.html