From Athlon Sports ...
Although the majority of games are won or lost in the trenches, newspaper headlines and sports talk radio rants are rarely focused on the big guys at the heart of every defense. So, the line lingo regarding defensive tackles is well deserved.
Talking heads love to throw around terms like “3-technique” or “2-gap” without any explanation following. And judging by their misuse when describing certain prospects, some of those draft gurus may or may not know what they’re actually saying — but it sounds good.
A defensive tackle’s work starts with gap responsibility — which spaces between opposing offensive linemen need to be clogged. The gaps are distinguished by letter — the A-gap is between the center and the guard; the B-gap is between the guard and tackle; the C-gap is between the tackle and tight end; the D-gap is just outside the tight end; and the E-gap is between the tight end and the wide receiver. There are two types of defensive tackles — those with two-gap and those with one-gap responsibilities.
Two-gap defensive tackles are usually bigger and stronger, occupying at least two blockers. This stuffs the running lanes, while also allowing the linebackers behind them to go unblocked, make plays and hog the glory.
In a 3-4 defense, Steelers two-gap All-Pro Casey Hampton (6’1”, 325 pounds) has rare athleticism for his size. Meanwhile, 17-year veteran Ted Washington (6’5”, 375 pounds) is an immovable mountain who can seemingly be effective by just leaning on two or more opposing offensive linemen. In a 3-4 defense, defensive tackles are better known as nose tackles and almost always have two-gap responsibilities. In order for a 3-4 to be effective, it needs a force of nature at nose tackle, which is very hard to find.
In a 4-3 scheme, the Super Bowl standard for two-gap defensive tackles is the retired Ravens duo of Tony Siragusa (6’3”, 350 pounds) and Sam Adams (6’3”, 350 pounds). The Goose and the Boston Lager were dominant two-gap tackles who dominated opposing offensive lines, allowing Ray Lewis to run untouched from sideline-to-sideline all the way to a Super Bowl XXXV victory.
One-gap defensive tackles are usually a sleek 290-to-310 pounds with quickness, speed and range. The more glamorous of defensive tackles, a one-gapper is more likely to sack the quarterback and less likely to serve only as a “blocker” for linebackers.
These days, whether the comparison fits or not, every young defensive tackle is the “next Warren Sapp.” And there is a reason for that. The Super Bowl XXXVII champ and 1999 NFL Defensive Player of the Year recorded 96.5 sacks over his 13-year career. Aside from being a notorious and controversial personality on and off the field, Sapp redefined the one-gap, “3-technique” defensive tackle.
The term “technique” is misleading. It is describing “where” a defensive tackle lines up on the field — not “how” a player does something (as the definition of the word “technique” indicates). But where a defensive tackle lines up is directly related to how he plays his position.
The numbering system starts at the center and moves out — a 0-technique defensive tackle lines up directly over the center; the 1-technique is between center and guard; the 2-technique is directly over the guard; the celebrated 3-technique is between the guard and offensive tackle; the 4-technique is over the offensive tackle; the 5-technique is between the offensive tackle and tight end; the 6-technique is inside the tight end or outside the offensive tackle on formations without a tight end; the 7-, 8-, or 9-techniques move farther outside and away from a defensive tackle’s normal responsibilities.
So, just in case you need the primer, there you go. Here is Wikipedia's page that also is useful.
Also, Speaking of Defensive Line, here is maybe the best edge DE or OLB in a 3-4 in the draft, Jason Pierre Paul. At 6'4, 270.