Another popular defensive alignment is the 3-4. This defense was used by Bill Parcels and his N.Y.Giants to win a couple of superbowls. From 2000 to 2004 it was also used by Bill Parcels' former defensive coordinator and now head coach of the New England Patriots, Bill Belichick, to win three more superbowls. This defense is now becoming quite popular again. The NFL is very much a copy-cat league.
In the 4-3 defense, you need two very large and athletic defensive tackles and two somewhat large and very athletic defensive ends. These guys are very hard to find. It seems there's about one excellent defensive end prospect in each draft, which is not nearly enough to go around. If you can't find a couple of good defensive ends, you're in for a long season of living and dying by the blitz.
One reaction to this has been to develop the 3-4 defense. In this defense, you need one really large nose tackle. This NT has to be a real monster of a guy, 350 pounds or so, because his job is to take on the center and one of the guards simultaneously on every single play. Then you get two more defensive tackles at around 300 pounds each, and play them up against the offensive tackles. All three of the defensive tackles have what is called two-gap responsibility. They are expected to hit the offensive linemen head on, and watch the play to make sure the running back doesn't come through on either side of them. Also, they're expected to hold their block so that the offensive linemen can't get out and block a linebacker.
In the 3-4 system, the linebackers are expected to make most of the plays. In the 3-4 system, the DTs play a more physical game as they are taking on one or two offensive linemen directly, play after play. Unlike the DTs in the 4-3, the DTs in the 3-4 are responsible for every single gap in the offensive line. Although the DTs get relatively few chances to make tackles or sack the quarterback, anything bad that happens is still ultimately their fault.
In the 3-4, you have four linebackers. Two of these guys are inside linebackers, and are expected to weigh roughly 240 pounds and be quite athletic. You also have two outside linebackers. These guys are sometimes called "tweeners," as they are in between the normal size of defensive ends and linebackers. These guys should weigh perhaps 255-265 pounds and also be quite athletic. Because these are linebackers, they tend to be faster than the heavier defensive ends. Their presence makes it much more difficult for the quarterback to roll out, as he will be rolling out directly into the path of one of these linebackers.
In the NFL there are many running backs who are incredibly athletic. O.J.Simpson was perhaps the first of these, but today there are perhaps a dozen of them. These guys are very shifty and hard to chase down. Defensive linemen simply can't do it. So the solution was to remove one of the DL from the defense and substitute a fourth linebacker. These linebackers have the speed to chase down such running backs. The 3-4 defense was motivated by two factors: the difficulty in getting good defensive ends, and the need to stop very fast running backs. This is the strength of the 3-4. The DTs keep the offensive linemen off your linebackers, leaving them free to roam the field and bring down runners.
Unfortunately, the 3-4 has a weakness. Three DTs cannot reliably collapse the five offensive lineman pocket on the quarterback, so it becomes much harder to pressure the quarterback. Traditionally, the 3-4 has struggled against the pass. The modern solution to this problem was worked out by Dick LeBeau, and it's called the Zone Blitz. In a normal blitz package, you have five or six guys rushing the passer. The remaining linebackers and defensive backs often played man coverage, which is relatively dangerous. Wide receivers in the NFL tend to be taller than defensive backs, so they often win the one-on-one battles.
In the 3-4, you select your four linebackers for the speed to drop back and cover a zone. Since there are four linebackers, the quarterback cannot guess which one of the four will rush on any given play. Whichever linebacker rushes, it's relatively easy for the other three to shift around a bit and fill in the zones. Sometimes the defense will rush two linebackers on the same side, so there are two defensive tackles and two linebackers attacking three offensive linemen. The remaining two linebackers again can quickly shift over to fill in the gaps left by the two who are blitzing.
If the 3-4 defense finds itself late in a game protecting a lead, there is no need to switch to a prevent defense. Since all four linebackers are skilled in coverage, you can just drop all four of them into zones, playing eight men in zone coverage. It's next to impossible to do this with a 4-3 defense.
The 3-4 defense can have trouble matching up against a very fast offense. It's important for the success of the 3-4 in these cases for the defense to play an extremely physical game, jamming the receivers hard at the line of scrimmage. In the 2002 superbowl, the Patriots looked like their defense matched up poorly with the numerous fast receivers of the Rams, but by playing an intensely physical game the Patriots managed to slow the Rams enough to eke out a win. Similarly, in the 2004 playoffs the Patriots hosted the Colts, and again it looked like a bad match up for the Patriots. But by making the Colts play a more physical game than they were used to, the Patriots managed to almost completely shut down the Colts offense. Had this game been played indoors in Indianapolis on carpet instead of outdoors on grass in cold weather, it's quite plausible the Colts would have found a way to exploit the lack of speed in the Patriots secondary.
So in summary, the more traditional 4-3 defense seems better suited to the modern pass-friendly NFL rules, while the 3-4 defense seems more geared to stop the run based offenses of the 70s and 80s. However, by mixing up the rushers and zone coverages to confuse the quarterback and by forcing the receivers to play a far more physical game than they would wish, the 3-4 has been successful at the highest levels, in spite of its apparent drawbacks.