The Defence - What are they trying to do?
The defense must stop the offense from getting the needed 10 yards for a first down and from scoring. The defense has two basic assignments, to tackle the runner and to break up the plays. A tackle is made when the ball carrier's knee touches the ground, in the grasp and hold of a defender, or his forward progress is stopped and he is unable to move.
As with the offense, the defense can use a variety of formations to achieve their goals. The defense is made up of "DEFENSIVE LINEMEN", "LINEBACKERS", and "DEFENSIVE BACKS", all of them trying to tackles the offensive ball carrier and give their team a chance to score.
Simply put, the defenders work to cover the offense while the offense tries to beat the defensive coverage.