Football includes three groups of players: the offense, the defense, and the special teams. Understanding the different positions in each category is key to understanding the game. The defense, like the other two categories, must put 11 players on the field. Here are the basic positions on the defense:

The defensive end lines up at the end of the defensive line. There usually are two defensive ends, one at either end. The job of the defensive end is to "contain" the running back on plays to the outside, tackling him or forcing him to the center of the defense by keeping him from turning the corner and running upfield. On passing plays, the end's job is to rush the quarterback.

The defensive tackle lines up on the interior of the defensive line. There usually are two defensive tackles. Their duties include stopping the running back on running plays, putting pressure up the middle toward the quarterback on passing plays, and occupying blockers so the linebackers have fewer obstacles in getting to the quarterback.

Nose Tackle

The nose tackle, also known as:the nose guard, lines up directly across from the other team's center. The primary responsibilities of the nose tackle are to stop the run and to occupy the offensive linemen, keeping them from blocking the linebackers.

The linebacker lines up behind the defensive linemen and in front of the defensive backfield. The linebackers are the team's second line of defense. Each team has two outside linebackers, who play toward the sidelines, away from the center of the field. In a 4-3 defense, the team has one inside linebacker, usually referred to as the middle linebacker. In a 3-4 defense, the team has two inside linebackers in addition to the two outside linebackers.

A cornerback is a defensive back who generally lines up on the outside of the formation, toward the sidelines. There are usually two cornerbacks. The cornerback generally is assigned to cover a wide receiver but also has responsibilities against the run.

Safety

The safety is a defensive back who lines up in the backfield between, but generally deeper than, the cornerbacks. His primary duties include helping the cornerbacks in pass coverage, but he also has responsibility on running plays. There are two safety positions: the free safety and strong safety. The strong safety plays on the strong side of the offensive team's formation, meaning the side with more offensive linemen. The free safety usually plays farther back and can roam the field as the last line of defense.

Nickel and Dime Backs

The nickel formation is used by the defense when a passing play is expected. It's called "the nickel" because five defensive backs are used instead of the usual four. Commonly this means only two linebackers are employed with the extra defensive back, called the "nickel back," who generally is faster and quicker to help protect against passes.

The dime formation also is used in anticipation of a pass, usually on third down with a long gain needed for a first down. It involves six defensive backs, meaning there is a "dime back" and one fewer linebacker or lineman.