How are American football and Canadian football different?

Size of the field 100 vs 110 yards
Number of players 11 vs 12
Number of downs 4 vs 3

ANSWER

Although the objective in both games is the same, there are several fundamental differences:

• In Canadian football, teams have three downs instead of four to move the ball 10 yards.
• There are 12 players on a Canadian team and only 11 on an American team.
• The Canadian field is 110 yards long (midfield is the 55-yard line), and the end zones are 20 yards deep. American football's end zones are only 10 yards deep.
• Goalposts are on the goal line in Canadian football and at the back of the end zone in American football.
• In Canadian football, there's a 20-second play clock. American football uses a 25-second clock (for plays that begin with a stopped game clock) and a 40-second clock (when the game clock is running).
• In Canadian football, more than one player can be in motion in the offensive backfield before the snap and can move toward the line of scrimmage. In American football, only one player can be in motion and must move parallel to or away from the line of scrimmage.
• In Canadian football, a team scores a 1-point "single" or "rouge" if it kicks the ball into the opponents' end zone and the opponent fails to return it. (Kickoffs that roll out of the end zone without being touched score no points.)
• In Canadian football, scrimmage kicks can be recovered by the offense at any time, from any point on the field. Players from the receiving team must be behind the kicker at the time of the kick to be eligible to recover, and the ball has to first travel 10 yards before anyone on the kicking team can recover. In American football, it's illegal to kick the ball beyond the line of scrimmage.
• In Canadian football, the defense must line up 1 yard off the ball at the line of scrimmage. In American football, the defense only has to be on the opposite side of the football.
• In Canadian football, when a ball is fumbled out of bounds, the last team to touch the ball gains possession. In American football, when a football is fumbled out of bounds, the ball goes to the last team that established possession inbounds.
• In Canadian football, each quarter must end on a play, with the result that many periods end with a play that starts with 0:00 on the clock. In American football, the quarter ends at 0:00, except if there was a defensive penalty.
• Following an opponent's field goal, a Canadian team can opt to either receive a kickoff or put the ball in play from its own 35-yard line. In American football, the team that scored the field goal has to kick off.
  

Those are the major differences, although there are other small ones.