The 3-4 defense uses three down linemen (two defensive ends and a nose tackle), along with four linebackers (two outside, two inside).
As you might expect, the roles of each position differ significantly from their counterparts in the 4-3, which uses four down linemen (two defensive ends and two defensive tackles) along with three linebackers (strong side, weak side and middle).
In the 3-4, the key defensive linemen is the nose tackle. An ideal nose tackle is large (up to 350 pounds) and capable of clogging the "A" gaps, or the spaces to his left and right between the center and the guards.
The defensive ends, meanwhile, typically are larger (between 280 and 300 pounds) than in the 4-3. In addition to pass-rushing, these players usually must be able to fill both the "B" gaps (between the guards and tackles) and the "C" gaps (outside the tackles).
The flexibility of the linebackers is what makes the 3-4 truly unique and dangerous. Conventionally, one outside linebacker is assigned a rush role, while the remaining guys offer run support or pass coverage.
However, defensive coordinators such as the Steelers' Dick LeBeau and the Packers' Dom Capers have become legends for mixing and matching roles using zone blitzes, which tend to rush five or six defenders and drop back the rest (including even nose tackles) into coverage.
The point is to confuse the offense into expecting a certain area of the field to be open, only to close it as the pass or the handoff is released.
Why use a 3-4 over the 4-3?
Some teams go with the strengths of the defensive coordinator, who may be versed in the system. Others might be compelled to make a change based on their existing personnel.
In the Raiders' case, both factors apply.