Cornerbacks, safeties and linebackers are generally the players used in coverage, and there are two types of coverages that they use – either zone coverage or man coverage.
Man coverage is pretty simple. A defender locks onto a receiver, running back or tight end and that's the player they're guarding. Zone coverage is a little more complicated.
Zone coverage designates a player responsible for a specific area, or zone, on the field. Linebackers usually cover the middle of the field, five to 10 yards off the line of scrimmage, while defensive backs guard the deeper sections of the field.
There are four main types of zone coverages teams use: cover 1, cover 2, cover 3 and cover 4. The number at the end reflects how many players there are guarding the deep part of the field. So in cover 1, there is one man guarding the deepest part of the field, from sideline to sideline. Cover 1 is usually employed when the offense is expected to run.
Cover 4 means that four players are responsible for the deep portion of the field, and they will each guard a quarter of the territory. This type of coverage is usually used when defenses are expecting a deep pass.
Defensive linemen can get involved, too. A zone blitz is when a defensive tackle or end drops off of the line of scrimmage and back into coverage, usually no farther back than five yards. Zone blitzes confuse the offensive line and can be effective against quick slants.
There are countless types of zone coverages that NFL teams use, but those are the building blocks many of the team's zone schemes are based. The Buccaneers, for example, are notorious for their "Tampa 2" defense, which revolves around a scheme based on cover 2 concepts.