"The" base front for Buffalo in 2013 was nominally a 4-3 front, and was a hybrid defense that incorporated both 1-gap and 2-gap principles. If you know what that means, you're ahead of the game. If you don't, I'll translate.
The Bills played what was essentially front with 4 down linemen. Mario Williams sometimes had his hand off the ground as a "linebacker" but the vast majority of the time he was in the game to play just like a 4-3 defensive end. He was going to rush the passer off the edge, and he was going to set the edge against the run. While some might focus on Williams as the "hybrid" part of this defense, whether Williams was in a 2- or 3- point stance was mostly inconsequential. The real "hybridity" in the scheme is how the Bills' players acted after the snap.
1-Gap vs. 2-Gap
To understand how a front could be a "hybrid" front between 1-gap and 2-gap principles, you must first understand what 1-gap and 2-gap principles are. 1-gap principles are easy.
In a true 1-gap defense, every defender has one gap he is responsible for and the unit stops the run by having every man tackle any hypothetical ballcarrier running through their individual gap. Against the pass, everyone who is designated to rush the passer will do so through his designated gap. This is simple, and defensive linemen love it. They don't have to wait and read the play, tthey just get off the ball and into their gaps. Here is a sub (read: NOT base) defense from Pettine:
1-gapping: every man has a gap, every gap has a man
2-gaping is unlike 1-gapping in many ways. Linemen do have to read the offense. They don't just get off the ball and into their gaps. And largely, 2-gapping linemen are trying to soak up blockers so that linebackers get to make the tackles, not trying to get off of blocks to make plays themselves.
Here is an example of how a defense might two-gap from Nick Saban's Alabama playbook:
There's a lot there, but let's focus on "Tech." "Run to," and "Run Away."
A "5 technique" is one where the defensive lineman aligns with his head on the outside shoulder of the offensive tackle, so for a "heavy 5" or "H5" the lineman would shift just slightly inside, almost head-up with the OT. He will align to be able to play ⅓ inside and ⅔ outside an offensive tackle. A 0-technique is head up on the offensive center.
As you'll note in the above example, all three defensive linemen are responsible for one of 2-gaps. The NT is responsible for the A gaps to either side of center. The Ends are responsible for the B gap when the running backs flow away from them, and the C gap when the backs flow to them. As you'll see below, the ends will work toward the flow of the backs, while the nose tackle works away from the flow of the backs:
This is done in an attempt to play to each player's strengths. The nose tackle is usually a mammoth player who is quick for his size. But when the offense runs the ball, he will probably not be able to out-quick the offensive linemen to the ball. The inside linebackers in a 3-4 are usually instinctive linebackers who are at their best seeking the ball and flowing to it. So, by sending the NT opposite the play, he is able to do what he does best: take on multiple linemen and keep them off of the linebackers. The linebackers are free to flow to the ball.
Bills' Hybrid Base
In Mike Pettine's hybrid defense, half of the line 2-gapped while the other half were 1-gappers. Below, you'll see NT Marcel Dareus aligned in a 0 technique, while DE Alan Branch is in a "heavy 5" technique. These are the two-gapping linemen. On the opposite side of center, you'll see DT Kyle Williams in a 3-technique with DE/LB Mario Williams in a "ghost 9" technique, outside of an imaginary ("ghost") TE. Those are the 1-gapping linemen.
Behind the line, Moats is aligned in a 40 technique, off the line and over the tackle. Alonso is in a 20 technique, off the line over the RG.
In this defense, the Bills are hoping to keep the RG from being able to leak up to block Alonso before Alonso can get in position to make a play. On this particular play, the back flow went to the defense's right:
Above, you can see Dareus attack the center and work his way toward soaking up the RG's block (away from flow). With the backs flowing to the right, Branch attacks the RT and works right. Lawson sets the edge on the 2-gap side. On the 1-gapping side of the line, Kyle Williams has his way with the guard, and look at Mario Williams' textbook form:
That Super Mario can stay that far under the offensive tackle, keep the tackle's arms off of him, and furthermore hold him up with one hand as he takes care of two gaps...wow. Williams has set the edge (he had the C gap in the scheme) and was so effective that he is now looking to help elsewhere.
Alonso and Moats lever the ball. Moats--the backer on the 1-gap side--gets downhill into the empty A gap in a hurry. Alonso slow-plays the ball, staying ready for a cutback because of the empty C gap to his left. The Bills theoretically have Leodis McKelvin covering that gap, but he's a cornerback who isn't exactly known for playing the run. Most offenses won't even scheme to block cornerbacks for good reason: McKelvin starts 7 yards deep and never gets much closer to the ball.
The back thinks about cutting back, but Branch has done his job on the backside of the play. You can see big #90 in the B Gap above. The run does go for 4 yards, but the Bills get a decent stop despite a -1 in the box situation, a missed arm tackle by Branch and Alonso turning his back on the play at some point in time.