In standard American football, there are four downs. At the end of the down cycle, there is a turnover on downs and the ball is given to the defense wherever the ball was last spotted. A down occurs when the ball is snapped and the player carrying the football is tackled or the ball is called dead. After a down is recorded, the game moves on to the next down. This new down corresponds to the next play.
In Canadian football, things are a bit different. They play with only three downs, whereas in American football there are four. The down system in both games works exactly the same: if the required 10 yards to reset the down counter are not met, the ball is given to the opposing team at the spot of the ball. With one less down, Canadian football is a much more pass-oriented game compared to American football, where running the football is still very much a viable option.