For years fans and pundits of football have tended to describe defenses by the personnel employed in their fronts, as in "are they a 4-3 defense or a 3-4 defense?"
This has become an increasingly out-dated and unhelpful way to describe what teams are doing for a variety of reasons. One reason for the uselessness of this terminology is that defenses are coming to be defined more by an outside-in paradigm with the coverages determining everything up front. What's happening up front is secondary to the defense after what they are doing on the back end.
Another is that there are "4-3" teams that will play their weakside or strongside 9-tech defensive end in a stand-up position and even occasionally drop him into coverage. Are they now a 3-4 defense despite utilizing the exact same front? Teams came to be better defined by whether they were asking their DL to be one-gap or two-gap players but then hybrid defenses started to take over in which some DL were two-gapping and others were not.
Finally there's the fact that with the rise of spread passing games at every level of football, most teams are rarely in 3-4 or 4-3 personnel packages anyways, instead playing nickel sub-packages. Since those nickel packages are the primary strategy for defenses, teams prioritize players that will fill the roles of those packages.
However, there are two nickel fronts that are coming to encompass the common packages of modern football in the way that the 3-4 and 4-3 served to describe what most teams were doing in the past.
The 2-4-5 front:
And the 3-3-5:
Each of these fronts tends to borrow more from one of the two dominant fronts of the 4-3 defensive era, the Over and Under shifted fronts. In college there are still teams that play their nickel packages with four down linemen, particularly quarters teams like Ohio State, Michigan State, or Kansas State that will ask their DEs to two-gap at times.
However, across the game these two fronts and personnel groupings are starting to define the main counters to modern offenses.
2-4 personnel vs 3-3 personnel
The 2-4 or 3-3 labels serve primarily to describe the types of players on the field. The 2-4 is going to feature only two true defensive linemen that are always going to be lined up with their hands in the dirt with an opposing blocker on either shoulder.
Then in place of defensive ends there will be two DE/OLB hybrid players on the edges in stand-up positions who specialize in attacking the edge and providing a pass-rush. The two inside linebackers behind these players are normal inside linebackers.
The 3-3 front features three true defensive lineman as the "defensive end" though they may line up on the edge, are big and sturdy enough to play interior gaps or face a double team. The nose tackle will generally be a standard big guy, with some exceptions.
The three linebackers behind the DL all need to be fairly versatile as well and although one of them might be the designated primary edge-rusher, each of them need to be competent performing as inside linebackers or blitzers.
Each style has certain requirements on the types of players that are required and which style a team chooses largely depends on if the defense has easier access or an easier time developing a couple of really athletic edge rushers and tackles as the 2-4 calls for or can find and develop the kinds of versatile, tweener players that make the 3-3 work.
The 2-4-5 is ultimately a defense of specialization as the main pass-rushers are going to be the two stand-up edge rushers. The defense deploys them on the edge because that's the easiest way to utilize a pure pass-rusher and they aren't asked to do a great deal other than control the edge and provide pressure. The defensive tackles will tend to specialize in clogging up the interior and helping collapse the pocket while the linebackers are running free as support players.
Without access to the kind of elite pass-rushers that can attack the edge and overcome an offense's best efforts at pass protection, the 2-4-5 is not a superior nickel package. It can also struggle against the run if defensive tackles aren't sturdy or the linebackers are deficient. However, it is the simplest and best way to allow big, fast, and powerful athletes to impact the game and attack the quarterback.
The 3-3-5, or 8-3, is more a defense of versatility and disguise that will require the DL to all be strong at filling interior gaps and ideally decent or good at collapsing the pocket. The linebackers are not specialists but "jacks of all trades" that can be transformed into superior pass-rushers by virtue of the system disguising where they are blitzing from.
Without versatile and intelligent players, the 3-3 is dead in the water, but when those pieces are in place it can pick on offense's weaknesses with greater precision and bring pressure from all angles.
2-4 vs 3-3 philosophy
At their hearts, the 2-4 and 3-3 are basically extensions of 4-3 Over and 4-3 Under philosophies. The Over front is generally the defense people are thinking of when discussing teams that "spin down" safeties into linebackers and linebackers into defensive ends.
The aim is to get speed on the field and allow it to run to the football with as little complication as possible. In the 4-3 Over that involves four down lineman but in nickel sub-packages where the pass-rush takes an even greater priority than it makes sense to allow the defensive ends to become more like permanently blitzing linebackers.
The 4-3 Under defense is one that's about filling interior gaps with big strong defenders, controlling the line of scrimmage, and dictating where the offense can go with the football. The 3-3 continues in that vein while acknowledging that it now requires fewer big bodies to control the line of scrimmage in a nickel package against spread out offensive sets with three or more receivers.
Either defense might employ or not employ two-gapping techniques by some or all of the DL but the only players that would do so would be the two interior DTs in the 2-4 and the three DL in the 3-3.
When it comes to converting to these packages from base 4-3 or 3-4 groupings, that can very much depend on the team's best personnel over any other factor.
Just yesterday in the NFL draft the Pittsburgh Steelers chose Bud Dupree from Kentucky, a 6'4" 270 pound edge athlete that will have obvious utility as an edge rusher in their nominally 3-4 defense.
Of course, you can't play a true 3-4 defense anymore with two players of Dupree's size and skill at outside linebacker or you'll be picked apart by spread formations that ask them to cover slot receivers and tight ends in space. It would be a waste of Dupree's pass-rushing and edge talents to ask him to drop in coverage as often as the Steelers would ask of previous OLBs in their zone-blitz driven defense and counterproductive regardless.
So do the Steelers maintain their zone blitz/cover 3 philosophy from a 3-3 front or do they maintain their preference for playing two powerful edge rushers by instead utilizing a 2-4 that is less well suited for disguising blitzers? That may well depend on how quickly Dupree comes along as well as the other OLBs on the roster.
The fact that Pittsburgh is supposed to be a 3-4 base team has less to do with their nickel package than which players they want on the field and what they want to do behind the front.
To take another example, the 3-4 oriented Alabama Crimson Tide will generally remove their 2nd outside LB/pass-rusher from the field in their "4-2-5" nickel package and instead play fronts that utilize 3-3 personnel.
This style of defense can often be as effective at rushing the passer as the 2-4-5 but unless the single edge rusher is a dominant player, the team essentially needs someone to pull double duty as both a good DE/NT/ILB and a dangerous pass-rusher.
Updating the language
Football punditry is desperately behind the ball in terms of using accurate and descriptive terminology to explain what's happening today on the football field. You'll often hear talking points about a team in the draft or in recruiting that revolves around finding ideal fits for a 4-3 or 3-4 base defense.
More often than not, talking points based on those terms will have very little value in describing what those teams are looking for and how they'll deploy players. In an age where the nickel package is really the base defense, teams will be defined more by whether they prefer to play three true defensive linemen or only two.
While someone can refer to the NFL as "THE National Football League" and it somehow catch on within a month as a way to make professional football sound like an important and serious business enterprise, updating schematic language is less likely to catch on as easily.
However, try thinking of defenses as either 2-4 or 3-3 when evaluating this current draft or your team's recruiting season and see if that helps your understanding of why your team makes their personnel choices.