It's all a matter of "Downs" don'cha know?
A down begins with a snap or free kick, and ends when the ball or the player in possession of it is declared down by an official.
Down is also an adjective to describe the condition of the player with possession of the ball after he has been tackled or is otherwise unable to advance the ball further on account of the play having ended (e.g., "He is down at the 34 yard line").
It may also refer to the ball after it is made dead in one manner or another. The line of scrimmage for the next play will be determined by the position of the ball when it is downed.
Each possession begins with first down. The line to gain is marked 10 yards downfield from the start of this possession, and the situation is described as "1st and 10". If the offensive team moves the ball past the line to gain, they make a new first down. If they fail to do this after a specified number of downs (four in American play), the team is said to turn the ball over on downs, and possession of the ball reverts to the opposing team at the spot where the ball was downed at the end of the last down.
When the offensive team has not yet made a first down before reaching the final down, the team faces a last down situation, where the team is forced to decide whether to either scrimmage the ball in an attempt to pick up the first down (this is called going for it [on fourth down]), or alternatively to kick the ball (either by punting or making a field goal attempt). Though statistical analysis of games suggest playing more aggressively is the better option, kicking the ball is typically seen as the safer solution; scrimmaging may lead to a turnover on downs, potentially giving the ball over to the other team with good field position.
Downing the player with possession of the ball is one way to end a play. Usually a player is made down when he is tackled by the defense. In the NFL, if the offensive player is touching the ground with some part of his body other than his hands or feet, then he is down if any defensive player touches him. In the NCAA, an offensive player touching the ground in the same manner is down, regardless of whether a defensive player touches him.
As Yul Bryner would say, "Etcetera, etcetera, etcetera!" L0L
This is a DOWN Marker: