A punt is a play in Canadian football and American football where the football is kicked down field to the oppossing team. If an offensive team has the ball too far away from the end zone to attempt a field goal, and is facing a fourth down (or a third down in Canadian football), and too far away from the first down marker, they may choose to punt the ball. This involves kicking the ball from a standing position after it has been snapped (usually a long snap). The purpose is to increase distance that the opposing team must advance the ball in order to score a touchdown or a field goal.
Once the ball has been kicked it changes possession to the other team. A player from the receiving team must be the first player to touch the ball after it has been kicked. Once a player from the receiving team has touched the ball, it is considered in play. The receiving team has the option of returning the punt, that is catching the ball and running it towards their goal. If the receiving team does not return the ball, the play ends, and the receiving team takes possession of the ball at the spot that the ball stop moving or went out of bounds.
If the ball goes out of bounds beyond the end zone, it is considered a touchback, and the receiving team gets to start their possession at the 20 yard line. If the receiving team drops the ball or touches the ball but does not catch it, it is considered a fumble, and may be recovered by either team. If the ball is punted out of bounds, it may be a penalty if the ball does not go at least a certain amount of yards. In this situation, if the penalty is accepted, the punting team will be forced to move backwards some distance of yards, and punt the ball again. The defensive team may also attempt to block the punt, by rushing the punt kicker instead of or in addition to trying to return the punt.