The defensive team in football is the team without possession of the ball, trying to keep the other team's offense from scoring. There are 11 players at various positions on the field for the defense on every play, just as there are for the offense. There are essentially three levels of positions on defense: the defensive line, the linebackers and the secondary. The defensive line are the positions that line up closest to the ball, directly facing the offensive line. In a classic defense, the defensive line positions are defensive end and defensive tackle.

The defensive tackles line up next to each other on either side of the football. Their primary job is to stop the run, and any pass rushing that comes from the position is a bonus. Typically, they take up blockers to allow other players a better chance to get to the quarterback. Those other players are usually the defensive ends, who line up outside the defensive tackles and rush the quarterback, while also helping to stop the run. In certain schemes calling for only three defensive linemen, a nose tackle will line up directly over the ball in place of the defensive tackles, with two ends flanking him.

The second level of defensive positions are the linebackers. There are different types of linebackers – outside linebackers, inside linebackers, and so on – but they all share common traits. Linebackers are required to rush the quarterback on occasion – called blitzing, stop the run and drop into pass coverage, usually against running backs or tight ends.

Players at any of the positions in the secondary must be fast. That's because they are required to cover wide receivers and prevent them from catching passes. Of all the positions in football, speed is most important in cornerbacks – the players who line up one-on-one with wide receivers.

However, speed is also important for the safety positions, which usually line up deep down the field and more in the middle of the field than cornerbacks. The strong safety usually plays closer to the line of scrimmage and helps support the run defense, while also sharing responsibilities in pass coverage. The free safety is more of a roamer, helping in pass coverage wherever needed.