As we have heard that the Broncos will now play more 3-4 than 4-3 - found this article explaining the difference between the two:
The Base 4-3
In a standard 4-3 defense, the line consists of two tackles and two ends. Where exactly they line up depends on the coach, the play, and the situation. But generally, the tackles take on the opposing guard and center, while the ends attack the opposing tackles and tight end.
What happens at the snap depends on the play and the philosophy of the coach. There are two basic responsibilities for defensive linemen: one-gap and two-gap. When playing one gap, the lineman attacks a space between two blockers. For example, one of the defensive tackles may try to penetrate the line by attacking the space between the right guard and right tackle. The defender is only responsible for what happens in that hole. When playing two-gap, the lineman engages his blocker, reads the play, and takes responsibility for anything that happens to his left or right. The same defender from the last example, playing two-gap, might engage the right guard head on. Once he knows how the play is developing, he'll know whether he has to move to the left or to the right to make a tackle or chase a play.
Whether they're playing one-gap or two-gap, the defensive linemen want to disrupt the blocking scheme and occupy offensive linemen. That frees the three linebackers to make tackles. Like the linemen, linebackers often have gap responsibilities. But when the four defensive linemen have forced all five offensive linemen to block them, the linebackers have it relatively easy: they only have to worry about the fullback and/or tight end getting out to block them, not a 320-pound guard.
On passing plays, the four linemen attempt to sack the quarterback in most situations. Again, the four-on-five mathematics is critical. Ideally, with defensive ends using their speed and tackles using their power, the defensive line will be so effective that all five offensive linemen will be needed to block them, and even then the quarterback will be in jeopardy. The defensive coordinator then has lots of options: he can send a linebacker or two on a blitz, knowing there is no one to block them, or he can commit the linebacker to coverage, knowing that the line will still be able to get pressure.
The one- or two-gap scheme is a gross oversimplification. Linemen can have many other responsibilities. They can slant, moving laterally to engage a blocker to the left or right of the one facing them. They can stunt or twist, wrapping around one another to confuse the blockers. Coordinators can create all sorts of variations from a base 4-3 front, but the goals are generally the same: disrupt blocking patterns, take away running lanes, harass quarterbacks.
In terms of personnel, the ideal 4-3 defensive tackle weighs close to 300 pounds but is quick-footed enough to shoot through a gap at the snap. Ends are lighter and quicker. Right ends, who line up against the offensive left tackle and attack the quarterback from the blind side, are usually the best athletes on the line: 275-pound monsters with incredible quickness and agility who can outflank blockers who are bigger and heavier. The middle linebacker in a 4-3 scheme must be as smart as he is athletic; his assignment on any given play may change from pursuing a running back to dropping into zone coverage, depending on what the offense does at the snap. The outside linebackers must be fast enough to chase ballcarriers from one side of the field to the other, and must also be big enough to do battle with an offensive lineman.
The Base 3-4
The key player in a 3-4 scheme is the lone interior lineman, who is usually called the nose guard or nose tackle. He lines up directly across from the center � on the center's "nose" � though he may sometimes shade over to the space between the center and one of the guards. He almost always has two-gap responsibility. His job on nearly every play: force two offensive linemen or more to block him.
The defensive ends in a 3-4 alignment have jobs similar to that of the nose tackle: they want to take up space, fill gaps, and occupy blockers. In a 4-3 system, linemen are supposed to occupy blockers, but they are also expected to free themselves to make tackles and sacks. In the 3-4, linemen aren't expect to make many sacks or tackles. Most of the playmaking responsibilities fall upon the linebackers.
The 3-4 system gives the defensive coordinator a variety of options. At the snap, he can blitz any combination of linebackers, and the offense doesn't necessarily know where the rush is coming from. Typically, one or both of the outside linebackers will attack the line of scrimmage, whether to pressure the quarterback or tackle a running back in the backfield. That leaves two inside linebackers to follow the flow of the play, pursue running backs, or drop into pass coverage.
The 3-4 alignment is popular now because it allows defenses to zone blitz effectively. The "zone blitz" is just what it sounds like: some defenders blitz, the rest drop into zone coverage. In a 4-3 system, zone blitzing is tricky: the linebacker or safety who blitzes leaves a zone unoccupied. Another player can take over in the unoccupied zone, but a) that defender is stretched pretty thin, with an extra-large zone to defend, and b) the quarterback can usually see what's happening. Many a smart quarterback has defeated a zone blitz by waiting for a linebacker to attack, then dropping a soft pass into the part of the field that the linebacker usually defends. But with an extra linebacker on the field, the defensive coordinator has more flexibility. The faster linebackers can rotate quickly at the snap of the ball, filling each other's zones.
Say the coach wants both the left outside and middle linebacker to blitz. Normally, that would leave a big space on the left side of the field, one the offense could exploit with a quick slant pass. But in the 3-4 alignment, there's an extra inside linebacker who can quickly slide into that unoccupied zone. The right outside linebacker can move over to cover the middle of the field. The zones are a little wide, and the defenders in coverage have a lot of space to defend, but the quarterback must assemble a jigsaw puzzle to figure out who's where, all the while bracing for the blitz.
The nose guard in a 3-4 system must be huge, strong, and have incredible stamina: he takes on two blockers per play, every play. Ends must also be bigger and more durable than the ends in a 4-3 scheme, though they don't have to be as fast. Pass rush responsibilities are handed to the outside linebackers, who are expected to be lightning-quick runners and ferocious hitters. The inside linebackers in a 3-4, like middle linebackers in the 4-3, have to be smart, athletic, and versatile.
Variations
The base alignments explained above are very broad outlines. In modern football, there are hundreds of variations of each base set.
For some teams, the 3-4 and 4-3 are almost interchangeable. The right end in a 4-3 may be a 255-pound defender who sometimes drops into coverage: how different is he from an outside linebacker? Or, the outside linebacker in a 3-4 may line up in a three-point stance on the line of scrimmage for much of the game: why not call him a defensive end?
Teams that want to use a standard 4-3 scheme often face a dilemma: there aren't enough great defensive ends to go around. Players like Julius Peppers or Jevon Kearse come along about once per year in the draft. Most college defensive ends are great athletes who weigh about 260 pounds: put them on the line against a 320-pound left tackle, and they'll be plowed under on most running plays.
Changes in offensive style have also forced evolution in defensive alignments. When an offense lines up with five wide receivers on first down, then two tight ends and two running backs on second down, what's a defense to do? It has to adjust, by using five or six defensive backs on some plays, and extra linebackers on others. The one constant is that the defense usually tries to attack the five offensive linemen with four defenders, but those defenders could be any combination of traditional tackles, ends, and linebackers.
Teams like the Steelers and Bills ran the ball so effectively last year that opponents switched to a 4-4 defense: four linemen, four linebackers. At times, the extra linebacker crept up to the line and into a three-point stance. Suddenly, the five-man defensive line had returned to the NFL, strategy coming full circle. That's the nature of the game, and that's why it's a source of endless fascination.
http://msn.foxsports.com/nfl/story/5...-3-vs.-the-3-4
