Below is a description of the four main coverages:
Cover 1: In this formation, only one safety is deep. He is usually in the middle of the field and his presence is largely precautionary – the defense is anticipating a run. The two cornerbacks are in "press coverage," which means they are matched up man-to-man against the opposing team's wide receivers. The second safety is approximately five yards off the line, preparing to cover the tight end.
"A disadvantage of this formation is the vulnerability of the single deep safety to the long pass play, but it's very effective against the run," explains Gillhammer.
Cover 2: A favorite of many coaches because the secondary can more readily adjust to different plays and audibles. Both safeties stay deep, while the cornerbacks cover the receivers on the line. It's advantageous because the safeties are prepared to help out on deep pass plays, but they can also come forward to defend against the run.
"There are two general types of a Cover 2 defense," explains Gillhammer. "There is the standard Cover 2 (described above), and there is also what is known as the ‘Tampa 2,'" made famous by the schemes of Tampa Bay Buccaneers defensive coordinator Monte Kiffin.
"In the Tampa 2, the cornerback jams the outside receiver toward the middle of the field, while the MIC, or middle linebacker, focuses on preventing him from penetrating up the middle," Gillhammer says. "The double coverage makes it more difficult for big-time receivers like Terrell Owens and Steve Smith to make the play."
Cover 3: A formation in which one safety is approximately 12-15 yards deep, while the cornerbacks are shallower at 10-12 yards, and the second safety is near the line in press coverage against the tight end.
"This zone defense also works well against the run, but a good quarterback and receiver tandem can pick it apart by throwing short passes ‘underneath,' or in front of the secondary, because it's difficult for bigger, slower linebackers to keep up with quick receivers," says Gillhammer.
Cover 4: This coverage requires all four defensive backs to stay off the line. They can play "tight," or five yards away, as they would in red-zone situations, or to stop the run, or they could play "loose," at 12-15 yards away, to prevent the deep pass play.
A coach might use this coverage towards the end of the game when he is trying to protect the lead and run out the clock.