Linebackers are important positions on every football team, as they serve in pass and run stopping, depending on the play. However, there are many specific linebacker positions that are dependent on the defensive formation.
The 4-3 and 3-4 defenses are the two main defensive base systems.
The 3-4 defense utilizes three defensive linemen on the line of scrimmage and four linebackers behind them.
When playing the 4-3 defense, there is only one middle linebacker. He is responsible for preventing the other team's center from getting down the field. When there is only one middle linebacker, it is known as the "Mike" linebacker. The Mike linebacker is also responsible for communication and organization of the defense-comparable to the quarterback's role on offense.
When playing a 3-4 defense, there are two middle linebackers. One retains the "Mike" label while the other is called "Ted." The Ted linebacker usually takes up blocks for Mike to make a play on the ball.
The weak side linebacker is on the opposite side of the field of the tight end. This linebacker is known as the "Will" (Weak side = Will, Middle = Mike, Strong side = Sam). The Will linebacker is often slightly quicker and smaller than the others because he is more likely to drop back and cover a slot receiver or running back in pass coverage.
In the 3-4, the Will linebacker might slide over next to Mike to add a fourth linebacker on the outside weak side, known as "Jack." The Jack linebacker is usually a pass-rusher or run-stopper, so they need to be larger players similar to defensive ends in size.