Either Cromartie isn't paying attention right now, or he's in zone coverage.
In the NFL, most teams employ a myriad of coverages in order to maintain a certain level of unpredictability, as well as to cater to their Defensive Back's strengths. Defenses utilize a medley of coverages including Cover 2, Cover 3, Man to Man, Cover 1, Cover 6, etc... The main philosophical difference between coverages lies in zone coverage and man coverage. Many Defensive Coordinators and Secondary Coaches have their own preference as to man vs. zone, varying from team to team, based on the talent available in the secondary, as well as the scheme of the Defensive System. However, all teams must use a combination of both zone and man coverage
Before every play the Quarterback and Wide Receivers must read the coverage they see, using this information to either call a play, run an option route, or just to see 'what's up.' The intent of this post is to help identify coverages that Defenses utilize both before and after each snap, as well as the purpose of each movement.
Reading the coverage before the snap is quite simple. Look at the Cornerbacks, if their backs are pointed towards the sidelines, they are in zone coverage. If their backs are pointed towards the endzone, they are in man coverage. Boom, you're done... Actually, not so fast. There are more details that can help you diagnose the nature of a corner's coverage.
Another indicator of coverage is where the Cornerback is lined up in relation to the WR. If he's lined up to the outside of the WR, then he is in zone coverage, while if he's lined up to the inside of his mark, he is in man coverage.
By lining up to the outside, zone corners try to funnel the WR's towards the middle of the field and friendly LB's and S's; where their help lies. Meanwhile, man corners line up to the inside of a WR in order to push their man outside -- toward the sideline. Man corners use the sideline as another defender in order to limit the amount of routes a WR can run. One of the worst possible things a man corner can do is allow his mark to cross his front and gain the inside leverage.
In addition, you can discern the nature of a CB's coverage by his depth from the line of scrimmage. If a CB lines up in zone coverage right on the LOS, then his responsibility will be guarding the flat. If he's in zone, but he's lined up 10 yards off the LOS he's probably watching the deep or intermediate zone, watching deep curls and crosses.
Furthermore, if a CB in man coverage is lined up on the LOS, he is probably going to press the WR in order to jam him at the line, so as to mess up the timing between the QB and WR. If the CB is in man, but he is lined up 10 yards or so of the LOS, he might be worried about the WR's speed, and could be trying to give himself a buffer, or he could be trying to discourage out routes or curls.
As their denomination suggests, Safeties are the last line of defense for a Secondary, and thus are typically playing in deep zones to prevent TD's and the like. However, if you see a Safety enter the box, he might be blitzing, or creeping up on a TE. A descending Safety could even indicate that the defense is lining up for Cover 1.
As you can see in this picture of a pre-snap read, the Cardinals are lined up in a Dime Defense with 3 CB's in man coverage, while the Free Safety is in a deep zone over top the X WR (Steve Smith) and the Strong Safety is lined up much closer to the LOS, in order to oversee that side of the field, or to cover the TE. From there you can determine the nature of what the Secondary is trying to do. On the weak side of the ball you can see a DB aligned in zone coverage for the far right third of the field, tipping Cover 3.
In today's NFL most secondaries 'sugar' their coverages, or hide their integrity of the coverage from the Offense, because if fans can read the defense, so can NFL QB's and WR's. A DB might be lined up to the outside, with his back facing the sideline, but at the last possible moment he shifts inside and downwards into man coverage. Or in the waning seconds before the snap he hurries into the box on a blitz. That flurry of action and movement is what moves us to the Post-Snap read.
Based on what a WR and a QB see in the pre-snap read, they may decide to alter the play completely, in order to take advantage of the coverage at hand. However, it's because of this that Defenses sugar their coverages, so to hide their intentions from the Offense, and hopefully coax them into throwing the ball right into the coverage. To combat this Offenses have installed a bevy of Option Routes --routes that give the WR an option to run two different routes based on the coverage he receives. This is why Offensive Coaches put so much stock into reading coverages, for if a QB and WR aren't on the same page, it can result in a very bad incompletion or even an interception.
Against a zone, WR's typically want to find holes in the coverage, whereas against man coverage, WR's are trying to gain separation. Run the wrong route against the wrong coverage, and you've effectively taken yourself out of the play.
Once the QB has read the pre-snap coverage and hikes the ball, the Secondary moves to complete their assignment.
In a zone, CB's will back pedal, all the time keeping an eye on the WR, as well as the occasional glance back to the QB. Zone corners will softly cover their man as long as he is in their zone, but once he leaves the zone, the corner drops the WR and shifts back into his domain. If the CB is in a zone he'll be facing the QB. The zone corner faces the QB in order to read the QB and make a quick break on the ball, as well as to provide run support against outside sweeps.
In man coverage you'll see the CB turn his back on the QB and shadow his WR all over the field. Rather than relying on the QB's eyes to make a break, in man coverage, CB's have to read their man's face, as well as his arms (if the WR is outstretching his arms, the ball is probably coming his way) for clues of an incoming pass. In man, the corners provide no help in run support.
Coverage is one of the many battles taking place in the great chess match between Offenses and Defenses, and will forever evolve as the game shifts. As fans, it's important to know what is going on as it happens in front of us, so that we can improve our understanding of the game.