The LA Rams, like all NFL Teams, organize the team under three separate banners: Offense, Defense, and Special teams. The offense is the easiest to track. Measuring an NFL offense involves direct statistics, or yards gained and points scored. In other words, move the ball, and score. Run the ball, pass the ball, score. Pretty straightforward.
The defense is measured in an indirect fashion. Its ultimate success is measured by how effectively it stops opposing offenses from moving the ball and from scoring. Since the defense is the antithesis of the offense, it is tracked with a different set of statistics.