A tackle in football is the action of a defensive player stopping the progress of an offensive ball carrier by bringing him to the ground. A tackle is also a defensive stat recorded when any part of the ball carrier's body, besides their hands or feet, makes contact with the ground as a result of the defender. When the ball carrier is tackled, the play is called dead and the next down will begin.
Players can use different methods for tackling depending on the game situation, each with its own technique. However, certain methods like leading with your helmet when tackling are banned from football and may result in a penalty. A quarterback sack is a special type of tackle recorded when the quarterback is tackled.
In the NFL, the term "down by contact" means that a tackle only counts if contact by the other team forces the player down. In other words, if a player with the ball falls to the ground but is not touched by anyone on the opposing team, this does not constitute a tackle and the play may continue. However, in college and high school football, if the ball carrier falls to the ground for any reason the play is called dead and the next down will begin.