Those of you know who read the comments know that I routinely talk about my favorite football article of all time. It was written by Dr B, former grand exhaled leader of Shakin' the Southland (Clemson's SBN site). In it, he covers the 3-4 in a way that I can only dream to touch. There's a lot of material there, and you really need to read it..
A lot of people barely even know what you mean when you say 3-4 or 4-3. If you're reading this article, chances are you know enough about football to know what those terms mean. However, just for the benefit of those of you who do not, 3-4 and 4-3 refer to the formation of the defense. They are the two major defensive formations in football, and teams usually use one of these two as a "base". When we say base here, we mean the formation the defense typically uses, but obviously situations may arise to force you into another one. Some examples of other defensive formations you've probably heard of are the 5-2, 46, nickel and dime.
The numberings refer to the number of lineman and linebackers in the play. So when someone says they are running a 4-3, they mean they are running a formation with 4 linemen and 3 linebackers. This is the most common defense you'll see in college and NFL ball. With a 4 man "front", you (hopefully) have 4 big guys up front to win the battle in the trenches, stopping a run before it even gets going. Sounds great, right? Unfortunately, the days of the ground and pound, run-first offenses (which I love) are quickly going away. The game is changing towards pass-heavy offenses, and if you want to run a 4-3, you're going to be ill-equipped to handle it. The typical answer may be to put a OLB on a third/fourth WR (and pray he's fast enough), or you may drop your safeties down and just hope you don't get burnt. Other times, a defensive coordinator may be forced to switch into a "nickel" or "dime" formation*. Now you're in a formation notoriously weak against the run. These solutions I've described either put you in danger or take you out of the comfort of using your base defensive package.
One other solution, which we saw a little of last year, is to feign a 4-3 and drop a fast DE back into coverage; plays of which bore the 3-4. More teams should run a 3-4 as it is, as it is very difficult to find a good DE who can do everything you need in a 4-3 (again, see here for a history lesson). The job of a DE in a 4-3 is pretty much "get to the QB". The problem becomes you need a guy who is big enough to beat the giant offensive linemen, but also fast enough to get to the QB (as opposed to a 4-3 DT which doesn't have to be as fast). There just simple aren't enough guys like that around. So a lot of teams run a 4-3, but really don't have the personnel for it. The 3-4 is a great answer to that, often "converting" your smaller linemen into a LB (more on that later).
Some people think of the 3-4 as a "pass defense", which isn't right. It certainly gives you better protection from the pass (because of the extra speed), but it's not a pass defense like the nickel or dime. That said, it should be obvious that you need those front 3 guys to be beasts. In particular, your NT better be wrecking havoc; think Vince Wilfork. A guy that the whole OL needs to know where he is at all times. The NT also has to be very smart and quick and able to identify a play before it happens. It's his job to cover those inside gaps off the center (you may know these as the "A" gap). Without a solid NT, the whole thing can fall apart. So while you need less linemen when you run a 3-4, the ones you have better be houses.
With only three guys on the line, it should be obvious that the linemen in a 3-4 aren't expected to make nearly as many sacks as their 4-3 counterparts. In fact, the whole idea is the 3-4 occupies 4 (ideally all 5) of the offensive linemen. It is from that which the greatest excitement of the 3-4 comes: the versatility of the linebackers. That is, you now have 8 possible places a blitz can come from - the 4 LBs and the 4 DBs. Having a faster defense means that you can have a LB take over in coverage on a CB / S blitz. The QBs and linemen will ideally not know where the blitz is coming from. If you've ever watched a Rex Ryan defense, you know what this is like. Finally, my favorite part of the 3-4 comes from the "Jack" LB (more on names below), who is a hybrid lineman-linebacker player. A guy who is big enough to get past blockers, but not big enough to really be an every-down lineman. "Jack" is also a LB who can effectively pass protect. He's a "Jack" of all trades.
While the 3-4 puts a lot of pressure on your defensive line, it makes sense for Vanderbilt to move to the 3-4. We simply have not had enough big guys to effectively run a 4-3. Stanford moved to it for similiar reasons - you're able to pick up lots of LB-ish size guys with decent speed, but not too many giants who are capable of being linemen. At the same time, you need your players to be above average in their "football intelligence" to handle a lot more roles.
Welcome to the 3-4 boys and girls.
That's about all I want to say for a general overview. I do, however, want to cover some nomenclature that we may start hearing. From television, you're probably more familiar linebacker positions referred to as ILB, ROLB, and LOLB. On occasion, although rarely, have seen LILB and RILB to try and differentiate between inside backs in a 3-4 system. That's not so much wrong as it is misleading. In fact, it's even wrong in a 4-3 to call the outside backs "left and right". LBs are better defined by where they lineup over the offensive line.
In a 4-3, life is easy. We call our LBs "Mike", "Sam" and "Will". Why is that simple? Well, because "Sam" covers the Strong side, "Will" covers the Weak side, and "Mike" covers the Middle. In a 3-4, obviously things are a little different. The typical setup is something like "Jack", "Mike" "Ted" and "Sam". Like before, "Sam" covers the strong side. "Mike" is still in the middle, but covers the weak side middle with his new friend "Ted" who covers the strong side middle. On the weak side, "Jack" sits. Again, "Jack" is typically a guy who's too small to be really on the line, but not really traditional LB material either. Confusing all these names further is that these naming conventions are not static, so pay attention if you hear something you're not familiar with.
Because I feel like I should reference it one final time, if you want to learn about all the different ways these guys line up, over/under, blitz packages how the 3-4 is used to disguise all the blitzes, you must read the Dr. B article on it over at Shakin' The Southland.
* A "nickel" has 5 DBs, leaving only 6 DL/LBs. A "dime" has 6 DBs, leaving only 5. How the defense arranges the men in the box is generally situational. Common nickel formations are 4-2-5 and 3-3-5; common dime formations are 4-1-6 and 3-2-6. As you may have guessed, the third number now refers to the number of DBs. It should be clear to see how while these formations are great for pass coverage, having less men down low makes you very vulnerable to the run.
AUTHORS NOTE: VTPhD by no means has any idea what he's talking about. He is not a football coach, and more damningly, didn't play football in high school. He mostly bases his football opinions on years of watching the his football teams fail him. That said, he can make a damned good pizza from scratch and would be willing to share a piece if you brought over a sixer or two. Also, we're watching the damned LSU game later. No, we are not watching the Rutgers game, fuck the Big 10, Rutgers sucks.