The linebacker was created with a single purpose: play a support role for the defensive line, much like a safety provides protect for a cornerback. Of course, as with anything, the position developed into much more than that.
The most important thing to remember when thinking linebacker is assignment, assignment, assignment. Much like a safety, they need to be quite versatile. Even in schemes that ask just specific tasks from their linebackers, versatility is a linebacker's best friend.
Historically, linebacker has been a position where, based on scheme, they can be vital stars of the defense or simple role players. Because their assignments can be so diverse in most defensive schemes, you will often find offensive lines adjusting their protection schemes based on what they percieve from the linebackers. A good LB corps can conceal their true intentions by masking it with something that percieves an entirely different assignment. While it sounds simple, it's no easy task, and requires some smart play from all three (or four) backers.
Defensive fronts also play a big part in how linebackers are used and what assignments are given to each one. The basic two base fronts, the 4-3 and 3-4, are more deeply explained in the Football Schemes guide.
Linebackers have a wide variety of skills that all types of linebackers have to varying degrees. While they can get by being really good at only 1 or 2 of these, the best linebackers are athletic enough to do it all with success.
- Speed. Not entirely vital, but a speedy linebacker -- or at least a linebacker with some burst to their initial movements -- can be great for nullifying off-tackle runs and or blitzing.
- Tackling. Linebackers need to be able to solo tackle, preventing larger gains.
- Covering. This goes for both zone and man assignments, though zone assignments at this position are more likely. They need to be able to locate the ball and contain their man or zone with authority. A linebacker good in coverage will be able to eliminate a majority of the easier over-the-middle catches with a good separating hit or simply by blocking the throwing lane.
- Jamming. Particularly the outside backers and especially the weak-side backer, a linebacker needs to be able to (occasionally) move into the slot and jam the receiver long enough to disrupt the timing or force the receiver into the corners zone.
- Recognition. As with most of the defensive positions, recognition can be vital. It can be the difference between -5 yards and a gain of 20 yards. A linebacker with good field awareness and recognition skills can put an end to a play before it even begins.
- Block-shedding. They need to be able to shed the occasional blocker to make plays,which is hard when you are usually outweighed by a good amount. However, playing with smart leverage and hand movement/placement can shed any tight end or make it possible to shed a guard off you who has made it to the second level. This can become vital in run stopping situations against tough offensive lines.
- Pass-rushing. Being able to shed a blocker sometimes isn't enough. Sometimes you need a linebacker to bring the heat and get behind the offensive line. This requires not only good leverage, strength and hand movement, but the genuine athleticism required to spin, swim, club and bull past offensive linemen.
4-3 Outside Linebackers
Outside linebackers in a 4-3 front are referred to as Will (for the weak, or non-tight end side) and Sam (for the strong, or tight end side).
Their roles will vary from defense to defense, but the genuine premise behind it is covering the area between the hash and the cornerback with run and coverage support based on their assignment for that play call.
While many defenses nowadays ask the same things of their outside linebackers (though not always the same assignments on the same playcall), there is a consensus view of the accepted strengths/weaknesses/roles of each. Some teams still use this formula, but as with everything else in the modern NFL, it's become much more diverse.
Sam linebackers are generally the bigger, stronger backers, better able to shed the block of a tight end. They line up over the tight end most of the time, and often times they are asked to jam and cover the end in passing situations. They usually get help from the strong safety in those cases
Will linebackers are generally the faster, more athletic linebackers. They are often called into zone coverage assignments and asked to cover or jam slot receivers in certain situations. They often cover the running back that attacks his side of the field first in man coverage, while covering the weak flat or hook/curl areas in zone coverage assignments.
4-3 Middle Linebackers
Middle linebackers in a 4-3 front are referred to as the Mike linebacker. They are usually responsible for receiving the defensive signals and relaying them to the rest of the unit. You could say they are the "quarterback of the defense."
Again, the roles they play vary from defense to defense. Generally, the Mike backer is assigned to protect the area between the hash marks and shut down the running back.
Depending on the scheme, they may simply be assigned to a particular gap, as would the outside linebackers. But the more popular use of a Mike linebacker is in pure run support from sideline to sideline, supporting both the defensive line and outside linebackers. Mid-field zones are also quite popular in passing situations. For all this, they are usually the most well-rounded and versatile linebacker on the field with the best ability to bring down a ball carrier 1 on 1.
Dealing with interior linemen from the offensive getting to the second level becomes a big problem with some middle linebacker, and it's become more and more of a neccessity for the Mike to be able to shed even the best blockers at times. A guy like Ray Lewis, as talented as he is, struggles at this aspect of the game and requires a large defensive front to ensure his freedom of movement.
3-4 Outside backers
Since the 3-4 is a scheme designed to conceal the fourth rusher, the outside linebackers in a 3-4 are often very pass-rush capable linebackers who possess the athleticism to be effective in zone and run coverages. This is quite a demanding position; not many pure linebackers are capable of being both a pass rusher and a run stuffer.
The weakside 3-4 backer is the one with the better pass rush ability. Playing opposite the tight end, they rely on the weakside defensive end to hinder the offensive tackle's ability to block them on the rush. They need to be quite athletic, speedy and capable of beating tackles to the edge to disrupt the quarterback. Outside of a pass rush assignment, they are generally responsible for covering the running back coming out of the backfield.
The strongside 3-4 backer is the one with the more well rounded skill set. They must be able to rush the passer successfully, but will also be required to play coverage against the tight end and support the run coverage. They will usually pass the coverage of a running back coming out of the backfield to the strongside inside linebacker.
Because 3-4 outside linebacker requires such pass rush talents while remaining strong against the run, college defensive ends in 4-3 schemes are often drafted in an attempt to convert them to a 3-4 outside linebacker. They usually have pass rush ability, and -- having played on a defensive line -- they are already instinctively concerned about stopping the run. In the case of pure speed rushers like Dwight Freeney, these guys do not project to this position because they are usually not as stout against the run as a bigger pass rusher with a blend of power and speed rushing skills.
3-4 Inside Linebackers
Responsible for playing the role of Mike, Will and Sam, the inside linebackers must be strong in shedding blockers, playing the run, and covering the middle of the field in zone. Occasionally, depending on play call, the weakside inside linebackers will be required to play the role of Mike while the strongside backer plays in pass coverage, and vice versa. They need to be pretty versatile for those reasons.
The strongside inside backer is responsible for the tight endl, sharing many of the duties of a 4-3 Sam backer while splitting the duties of the Mike backer with the weakside linebacker.
The weakside inside linebacker will occasionally be responsible for maintaining a strong mid-field zone and has many of the duties that Will in a 4-3 would. He, too, splits the duties of the Mike backer with the other inside linebacker.
Summarizing it all
Linebackers are the safeties of the front 7. Depending on the scheme, they will have many different roles in a defense, but it all revolves around supporting the efforts of the down linemen and, occasionally, reinforcing the actions of the secondary. They need to be pure football players, capable of doing whatever is asked of them while maintaining a strong skill set, one that reflects what their team's scheme requires from the position.
While some defenses will require their linebackers to take a passive "wait and see" role and hold their ground, breaking big plays before they happen and giving up ground in small bits, others will ask their backers to be aggressive and attack plays with blitzes and proactive run coverage. Each of those schemes and every one in between require different forms of effort from each linebacker. You always need the right backer for the job.