There are 3 types or positions of defensive players. Defensive Linemen, Linebackers and Defensive Backs. The Defensive Linemen line up in a hunched over position nose to nose with the offensive linemen. Their purpose is to immediately disrupt the offenses blocking schemes and ultimately to tackle the ball carrier, if a running play is called. A sack is a play where a defensive player tackles the quarterback behind the line of scrimmage.
Outsiders tend to be faster and have to cover more territory as well as rush the quarterback.
The Linebackers line up just behind the defensive linemen. They are multi purpose players, sometimes taking the role of a lineman in tackling the ball carrier or pressuring the quarterback, and other times dropping back to help defend against pass plays. 240 is about the going average. There are two types of linebackers, Inside and Outside Linebackers. Like their defensive linemen counterparts, Outsiders tend to be faster and have to cover more territory as well as rush the quarterback.