The 4-3 defense in football is the premiere formation. It is by far the most common seen throughout all levels of play. The four implies there are four down defensive linemen (DL). The three designates the number of linebackers (LB). This leaves four defensive backs (DB). It is a common formation due to its great balance. There are enough people on the line to hold a run and also attack on pass plays. The linebackers are able to step up on the run and back off on passes. Meanwhile, there are enough defensive backs to prevent getting burned deep by fast receivers, but not too many that an offense can manhandle the defense by running the ball. It is a good base from which to adapt to alternative formations depending on the situation. Because of this, it is probably the best defense to teach younger kids who are just learning the game. It focuses on the fundamentals on which everything else builds.
The four defensive linemen consist of two defensive tackles (DT) and two defensive ends (DE). The defensive tackles tend to be slightly larger than the ends. This is because it is important for them to not be pushed off the line right off the start of the play when the offense is trying to run the ball. Players geared towards being run stoppers are particularly larger. On the whole, defensive linemen, however, tend to be smaller than their offensive line counterparts. The reason for this is the defensive linemen still need to be fast enough to catch ball carriers and tackle them. The defensive ends tend to be slightly smaller to gain extra speed. On pass plays, they like to be able to rush off the outsides and run around the offensive tackles (OT) to get at the quarterback (QB). They also have to have enough speed to prevent ball carriers from beating them to the outside.
The three linebackers consist of a middle linebacker (MLB), left outside linebacker (LLB/LOLB), and right outside linebacker (RLB/ROLB). Linebackers tend to be smaller than defensive linemen to allow them increased speed for pursuing ball carriers, blitzing, and defending passes. Again, the inside, MLB, linebacker tends to be larger and slightly slower than the outside linebackers for similar reasons.
Sometimes teams will play it where one outside linebacker (OLB) will always play the strong side (SLB/SOLB), and the other will always play the weak side (WLB/WOLB). The strong side is in reference to the offensive formation. The side of the offensive formation with the most players on it is known as the strong side. It is possible for the offensive formation to be balanced. In this case, the strong side is typically defined as the wide side of the field. If the ball is also in the middle of the field, then the details can picky on how a team might want to arrange its strong side and weak side linebackers.
Typically, the SLB is the better player or the slightly slower player when compared to the WLB. This is because offenses tend to run plays to the strong side of the formation more commonly than naught. Thus, it is nice to have a defense’s better players closer to the play or to allow the quicker players to run down a play from the backside. Otherwise, if an offense beat the front side of a play, the direction the play is going towards, the play would go for more yardage because the backside would take longer to chase down the ball carrier.
Finally, the four defensive backs almost always consists of two cornerbacks (CB) and two safeties (S). The safeties are usually defined as a free safety (FS) and a strong safety (SS). Here again, the strong safety tends to take the strong side as defined earlier. The strong safety tends to play a couple yards closer to the line of scrimmage (LOS), where the ball is at the start of the play, and plays more aggressively. Meanwhile, the free safety’s primary responsibility is to let no one on the offense get past him. He must cover the deepest man on any pass plays and is the last line of defense on any running play. The cornerbacks are usually slightly slower than safeties which is why they are not asked to prevent anyone from getting behind them for the most part. However, being a defensive back, they still need to be faster than the linebackers since their first responsibility is defending the wide receivers (WR) on passing plays. Cornerbacks also get to step up on runs to the outside.
Those are the basics of the 4-3 and positions contained within. Obviously, the exact specifics of each player and position depend on the team, plays run by the defense, and situation in the game. If it were so straightforward, football would not be the complicated, tactical game that it is. There is a reason why football is the most popular sport in the United States. The 4-3 is simply the most fundamental defense in the game.