I learned the rudiments of football through playing it with friends on snow-covered schoolyards in Littleton, Colorado and by sitting with my dad watching the Broncos on those Sunday afternoons when they were on television. I quickly learned that the defensive line had guys called defensive ends (DE) and defensive tackles (DT) and that their primary purpose was to tackle whoever had the ball, preferably while he was still in the backfield. I became acquainted with the difference between a 4-3 alignment and a 3-4 one and it seemed quite simple: the difference was in the number of defensive linemen who started in a 3-point stance.
I came to feel fairly confident talking with family and friends about the DE, the DT and even the nose tackle (NT). I felt like I had a handle on where they lined up and what their responsibilities were (though I must admit, I had to scramble a bit when the Broncos ran a 5-2 for part of 2009). Then I came of age when I became a reader of MHR and was exposed to the wealth of knowledge available in the MHR University articles. I started seeing terms I'd never heard before. Most recently, I have been fascinated by the statements in the MSM as they scout potential NFL players by saying something along the lines of "Joe Player is a very good 3-technique lineman" or "Jeff Player is a better 5-technique than 3-technique lineman."
Now I thought I knew all of the positions along the defensive line, so I was left thinking, "What in the name of the football gods is a "3-technique or 5-technique lineman?" If you have the same question, take a jump with me and see what we can discover.
In very general terms, your defensive ends tend to be lighter players (if you can call men who weigh in the 285-300 pound range light) who are expected to be faster than the defensive tackles and better at slipping off the blocks of 300+ pound linemen. The defensive ends tend to line up outside the ends of the offensive line, since they want to avoid blocks, if possible, in order to get into the backfield quickly. Defensive tackles tend to be heavier than defensive ends (often in the 305-360 pound range).
It is important to remember that the defensive linemen's specific assignments will vary based upon the defensive play that has been called and upon how they read the offensive play -- based on the formation shown by the offense and movement of the offensive players prior to the snap. Typically the defensive tackles are tasked with tying up blockers, getting into the backfield and making sure that the running backs are not able to get through their section of the line.
What may be less understood by many fans is how the defenders view the offensive line. In a typical offensive formation, the offense will set up in an alignment similar to this:

The basic offensive line is the center (C), flanked by two guards (RG and LG), who are in turn flanked by two tackles (RT and LT). The side with the tight end (TE) is referred to as the "strong" side and the opposite side of the offensive line is referred to as the "weak" side. This seems so simple that you might wonder why I mention it. It's largely to make sure that we're all following the same flow of thought.
However, when we consider how the defense views this set up, a couple of things begin to emerge: the defense will not only see the players, but they will also see letters and numbers. The space on between the center and either of the guards is referred to as the "A Gap." The space between the guards and the tackles is the "B Gap." And the space between the TE and the nearest tackle is the "C Gap."

Note that there will only be a "C Gap" on the strong side. Obviously, then in a two tight end formation, there would be two C Gaps. When you hear an announcer referring to the "gap assignment" of a player, this is what he is talking about -- the defender has been tasked with guarding a particular space between two of the offensive linemen.
Now here is where it gets interesting (and more difficult, I would imagine, for the defenders): each offensive blocker has two numbers associated with him, based on his "inside shoulder" (the shoulder closest to the C) and their "outside shoulder" (the one farthest from the C).

This is where the concept of "(#)-technique" comes in. At its most basic level, the term refers to where the defender initially lines up. A so-called "5-technique" player simply refers to a defender who initially lines up over the outside shoulder of an OT while a 3-technique player would be one who initially lines up over the outside shoulder of an OG. In a typical 4-3 defense, the DTs tend to be "3-technique" players and the DEs "5-technique." In the typical 3-4 defense, the NT tends to be a "1-technique" player while the DEs are either 4- or 5-technique players. However, telling exactly where the player has lined up (and hence what "technique" he's playing) is a very fine line since there is not much of a difference between the outside shoulder of a guard and the inside shoulder of the tackle next to him.
Troy Hufford reminded me when we were discussing this concept that it is important to remember that -- depending on the given defensive alignment and the offensive set at the start of the play -- a given defensive lineman may be moved around to create a mismatch and hence end up playing more than one technique during the course of a game. It has become popular in many circles to evaluate a defensive lineman as a good "#-technique player." In our discussion, Troy made the following comment about that:
I'm not sure that you have a certain "player" that is a set technique. That's like asking if a basketball player is a cross over dribbler. Yes, cross over dribbling is probably a technique that the player is good at, but it's not the only skill that he needs to have. They should be able to do cross over dribbles, between the legs, around the back, and so on to keep the defense from stealing the ball from you . . . Because of that, a defensive lineman should be able to play numerous techniques (not just a 3-technique. 5 techniques are normally pass rushing defensive ends. You'd probably see it a lot from Jared Allen. Again, this can be changed depending on the offensive formation and down and distance. 3rd and long situations would probably call for the defensive ends to be in a 6 (outside shoulder of the tackle) or 7 (inside shoulder of the TE) technique, trying to get around the corner of the offensive formation . . .
Well, if you look at a guy like Suh, he lines up everywhere on the inside. Sometimes he's a 1 technique. Sometimes he's a 4. If you're a good player, you're a good player.
#-technique then is a reference to where the player lines up based on the defensive alignment, and whether the offensive play is a passing play or a rushing play (and this is not including any moves -- often called stunts -- being used by the defenders to confuse the blockers). At its simplest level, a 4-3 defense has two alignments for the defensive line: a 4-3 Over and a 4-3 Under and a 3-4 defense has one base alignment.
In a 4-3 Over, the larger of the two defensive tackles will line up over the "weak side" shoulder of the center (that is, the shoulder to the side of the offensive line that does not have a tight end). The other DT will be lined up in a 3-technique position on the strong side. The DEs are lined up in a 5-technique on the weak side of the o-line and the other will line up outside the TE. In this alignment, the d-line is pitting their strength against the strength of the o-line. Both of the DTs could potentially be double teamed in this alignment. This is arguably a better defense against a running play.
In the 4-3 Under, the alignment of the two DTs is reversed, with the larger one lining up over the strong side (the side with the TE) shoulder of the center and the other DT lining up in a 3-technique on the weak side guard. This pits the strength of the defense against the weaker side of the offensive line. In this alignment you typically have three defenders lined up against three blockers so the chances of a double teamed block are reduced. This is arguably a better defense against a passing play.
On passing plays, the primary job of all four down linemen is to reduce the ability of the quarterback to successfully pass the ball. The DEs will attempt to do this by sprinting to the outside of the offensive tackles. The OTs will typically retreat into the backfield to blunt this charge. The DTs typically will try to penetrate into the backfield through the A and B gaps, and failing that, will attempt to drive the olinemen blocking them backwards so the QB has nowhere to step up if the DEs are able to penetrate.
On running plays, the DEs will still attempt to penetrate into the backfield, but now their primary goal is to force the RB towards the middle of the line. The DTs job is to put their bodies into the A and B gaps so the RB does not have a hole to run through. Ideally, the defense would like to see one of the defensive linemen tackle the RB before he crosses the line of scrimmage, but if they can't accomplish that, they will try to slow the RB down enough that another defender -- a LB or a DB -- will be able to make the tackle. The downside to the 4-3 defense is that on any given play, it leaves one A gap, one B gap and one C gap open, which means the linebackers have to step up to help against the run. This is the primary weakness of the 4-3 -- it is often not as strong accused of not being as strong against the run as against the pass.
In a 3-4 defense, the concept of #-technique takes on a role of lesser importance, since typically all three of the defensive linemen line up directly across from the C and the two OTs. The 3-4 typically uses a very large DT, usually referred to as a nose tackle (NT) -- a player in the 350+ range -- whose job is to tie up the center and one of the OGs on every play. His primary responsibility is to control both of the A gaps and keep the RB from penetrating up the middle. The typical 3-4 also uses two 300+ pound DEs (also often referred to as DTs, depending on the defensive terminology being used by a given team). In the basic 3-4 set, these two DEs line up directly across from the OTs and are tasked with clogging up the B and C gaps on their side of the line, again denying the RB a hole to run through. A secondary responsibility for all three linemen is to keep the blockers tied up so that they cannot release upfield to take on a LB or DB. Typically, the NT and DEs in a 3-4 are not going to be the players who make the most tackles and sacks since they are pressuring a five-man line with just three players.
Whereas it has often been suggested that the 4-3 is a stronger defense against a passing offense, while a 3-4 is typically stronger against a rushing one and that the 3-4 requires the defense to make a more creative use of their LBs and DBs to get pressure on the QB, the reality is perhaps a bit more complicated. A 4-3 would tend to be stronger against an inside running attack while a 3-4 would be stronger against an outside one. It is also important to remember that there are a great many variations within these basic alignments depending on the exact defensive system designed by any given team.
The Broncos have run both defenses in the past: Dan Reeves ran the 3-4 and took the Broncos to four Super Bowls. Mike Shanahan shifted to the 4-3 and won two Super Bowls. Josh McDaniels changed back to the 3-4 in the hopes of establishing a New Englandesk dynasty in Denver. From what I've read, John Fox will be changing it back to the 4-3. It will be interesting to see which of the current players are able to make the switch, which will traded or released and who Denver will find to fill in the holes.