The role of a cornerback (or safety) in coverage is to act as a "shadow receiver" for forward passes. He is assigned either to a specific offensive receiver (in man-to-man coverage) or within a certain zone of the defensive playing field (zone coverage.) The rules of legal coverage allow both offensive receivers and defensive coverage to have a fair and unimpeded chance at catching a thrown ball. A defensive player in coverage may disrupt the ball in flight or during an attempted catch, but may not push or strike the opposing player in order to impede his ability to catch the ball. Violation of the rules by either the offensive or defensive player is referred to as pass interference and the offending team is assessed a penalty.

As a critical element of any defensive secondary, the cornerback position demands players have speed, agility, and exceptional instincts. Additionally, a cornerback's skillset typically requires proficiency in backpedaling, shedding blockers, man-to-man coverage, disrupting pass routes, jamming receivers at the line of scrimmage, reading the quarterback, understanding zone coverage, and open-field tackling.

Most modern NFL defensive formations use four defensive backs (two safeties and two corners); CFL defenses generally use five defensive backs (one safety, two defensive halfbacks, and two corners).

A corner's responsibilities vary depending on how the defense assigns protection to its defensive secondary to handle the offense. In terms of defending passing plays, a corner will be typically assigned to either zone or man-to-man coverage.

Strong safety

The strong safety has a lot of responsibility on the defensive side of the ball.The strong safety tends to be a bit larger and stronger than the free safety. He is tasked to handle the "strong side" of the offense, the side where the tight end lines up. The strong safety tends to play closer to the line and assist in stopping the run. He may also be responsible for covering a player, such as a running back or fullback or h-back, who goes in motion in the backfield and then out for a pass.

Free safety

The free safety tends to be smaller and faster than the strong safety. His job tends to be to stay back a bit, watch the play unfold, and follow the ball. On pass plays, the free safety is supposed to close near the receiver by the time the ball gets to him. Offenses tend to call play action passes specifically to draw the free safety closer to the line to stop a long run. If the offense puts a receiver in the slot, then the free safety may be called upon to cover that receiver. Free safeties occasionally blitz as well. When this happens, the pressure is very severe since a blitz by a defensive back is not usually anticipated.