3-4 Front Defense
There are many different plays a defense can run out of a 3-4 set, but overall it includes some common qualities. The basic principle behind the 3-4 is that the three defensive linemen will take up as many blockers as possible, allowing the four linebackers to cover the remaining gaps and make plays on the ball.
In the college game, where the spread offense has become immensely popular, a specific variation of the 3-4 has become the premier defense for stopping the spread. It is known as the 3-4 Tite Front and is designed to muck up spread sets that rely on zone reads.
The three defensive linemen in a 3-4 set consist of two defensive ends and one nose guard (aka nose tackle). In a traditional 3-4 front, all three defensive linemen will use "two-gap" techniques. This means that their goal is to take up both gaps on either side of an offensive lineman in front of them.
The main variation between types of 3-4 sets lies in how the defensive line attacks the offense. The "one-gap" variation means that the defenders will each attack one gap, rather than attempting to take up two.
The four linebackers in a 3-4 have very distinct roles. More often than not, the two outside linebackers will line up next to the rest of the defensive line along the line of scrimmage. The linebacker on the weak side of the formation (the side with no tight end) will have to contain the outsides.
There are no specific roles for defensive backs in the 3-4, but the personnel group is almost always the same. Because there are seven players "in the box" (near the line of scrimmage), that leaves four defensive backs: two cornerbacks and two safeties.