Currently the Jacksonville Jaguars run a 4-3 defense that uses a lot of zone coverage concepts. There is a possibility the team changes to a 3-4 defense, depending on the coaching staff. I explained the nuances of a 3-4 defense here earlier, so I wanted to make sure to explain the different looks in a 4-3 defense, especially the look the Jaguars have employed the past few seasons.
Once again for reference is the defensive line technique numbering chart:
As you can see, the numbering system starts with the 0-technique, which lines up directly over the nose of the center on the offensive line. From there, the system expands all the way to the 5-technique which plays the outside shoulder of the offensive tackle.
Each odd number on the numbering system plays a shoulder of the corresponding offensive lineman whereas the even numbers play over the offensive lineman, directly. In a standard 4-3 front, or the 4-3 "stack" you're going to have your defensive line typically line up with the strong side end (LDE) at the 5- or 9-technique, a defensive tackle at the 3-technique, a defensive tackle at the 2-technique, and the weak side defensive end at the 5-technique.
With the 4-3 "stack" formation, you'll have your three linebackers fill the gaps between your lineman, so you'll have the middle linebacker (MIKE) between your two defensive tackles with the weak side (WILL) and the strong side (SAM) between each defensive tackle and defensive end.
Here is what the 4-3 "stack" generally will look like:
The Jaguars defensive line is lined up in an odd front in this shot, but you get the picture. Each linebacker is between a gap on the defensive line.
Now, there are different variations of the 4-3 defense, depending on how a defense shades. A team like the Jaguars commonly plays what's called a 4-3 "under" defense with a Cover-2 shell in the back end.
Here is what the 4-3 "under" generally will look like:
With the "under" look you have Jason Babin playing the 9-technique (since he loves the "wide-9") with the defensive tackles playing a (from right to left) a 1-technique and a 3-technique with the strongside end playing what looks like a 4-technique but is probably a 5-technique. In the "under" look you also have the strongside linebacker (SAM) basically on the line of scrimmage with the middle linebacker (MIKE) between the two defensive tackles and the weakside linebacker (WILL) between the defensive tackle and weakside defensive end.
As far as the 4-3 "over" look, you'll typically have the SAM lined up between the strongside end playing a 9-technique and the defensive tackle playing a 3-technique. The MIKE will be lined up between the defensive tackle playing the 1-technique and the weakside defensive end playing the 5- or 6-technique (essentially he's on the butt of the defensive tackle). Finally the WILL will be lined up shaded to the right of the weakside end.