There are a huge number of ways to align a defense on the field. Some defenses, like the 5-3 are odd defensive fronts, which means that there are an odd number of defensive linemen. (Usually this means that the center has a defender on him.) Other defenses, like the 4-4, are even and use an even number of defensive linemen.
Whatever defense you choose to use must be dictated by your personnel, the types of offenses you will face, and the skills you are able to coach. A powerful, dominating athlete will make a good nose tackle, and allow you to use odd defensive fronts. A number of aggressive, quick players will allow you to play multiple linebacker systems like the 4-4 and the 5-3.
In every discussion of defensive football however, reference is made to specific gaps and specific techniques used by defensive players to determine their alignment against a particular offense. Simply put, a gap is a space between offensive linemen, and a technique is a description of how the defender should align on his offensive counterpart. Since the rules require the offense to have a minimum of seven men on the line of scrimmage, there will be a minimum of eight gaps.
Figure 1 shows the specific nomenclature used to describe the offensive gaps. Gaps are given letter names, and named from the inside out. Thus, the center/guard gap becomes the "A" gap, the guard/tackle gap the "B" gap, and so on.
Figure 1: Defensive gaps.
When a defensive player is aligned in the gap, he is considered not aligned on any offensive player. Generally he will have responsibility for that gap, and for aggressively defending any offensive plays that attack it.
Some defenses, like the 5-3, are slanting defenses. Slanting defenses are designed to confuse the offensive blocking schemes by aligning in a position to attack either gap. When the football is snapped, a slanting defender generally strikes to either side of the offensive player he is aligned on. A slanting defender is called a "two gap" player.
Unfortunately, it's just not enough to say that a defensive tackle is aligned "on" the offensive tackle. A youth offensive tackle may be two and a half feet wide in his shoulder pads, and that is a lot of room for error on game day. To solve this problem, and to make our defenses more easily able to counter the strength of an offensive formation, the defensive players' technique is used.
Figure 2 demonstrates defensive techniques. Note that 2, 5, and 8 techniques are "head up" on the offensive players. 1, 4, and 7 techniques are "inside shade" and 3, 6, and 9 techniques are "outside shade".
Figure 2: Defensive technique.
You may already be somewhat familiar with a similar numerical system that was developed by "Bum" Phillips in the earlier days of football. Please note that this numerical system is different than the one Coach Phillips used. The numerical system used here is taken from the Tomales High School Defensive playbook, and is used because it is both simple and intuitive.
Generally defenses are numbered. The first digit is the number of defensive linemen in the system, and the second is the number of linebackers. Thus we have terms like the 4-3, 5-2, and 4-4. However, there are times when this description is invalid. The "46," for example, was named after the jersey number of the strong safety that Buddy Ryan used in a tight linebacker position. The player was Doug Plank, and his jersey was numbered 46. The system does not use four linemen and six linebackers.
After the ball is snapped there are a number of defensive techniques that are used to distrupt the offensive blocking. You'll hear terms like "Cover three" or "Cover one" bandied about. These are brief descriptors of pass coverages. A "Cover three" means that there are three deep pass defenders (generally a free safety and two corners.) A "Cover one" means that there is one deep defender playing "center field." This type of deep coverage is generally used when the defense is running a man-to-man coverage system underneath the zone.
Typically teams use man-to-man coverage when they blitz. Blitzing is generally an anti-pass tactic used to overpower the blockers and pressure the passer. Because there are already more passing zones to cover then there are defenders to cover them, blitzing usually forces the defensive team out of man coverage.
Sometimes I fall into old habits and use the terms "blitz" and "stunt" interchangably. This is not wholly accurate. "Blitzes" are defensive tactics that involve the defensive backfield (linebackers and defensive backs) charging through a hole. It can and should be used both to disrupt the pass protection and interfere with run blocking. Blitzing players must be carefully coached to break through the line under control and locate the football; not get hung up on attacking the quarterback. (I never, by the way, blitz my defensive backs.)
Stunting is a different technique. Stunts involve the linemen. Figure 3 demonstrates a simple "cross" stunt involving the interior tackles. This is an advanced technique that can be difficult to teach at the lower levels.
Figure 3: Cross stunt from basic 4-4.
Some coaches, myself included, occasionally refer to the stunts as blitzes and vice versa, as long as you're aware that it involves a change from the basic assignments of the defense, there isn't much to worry about.
Occasionally you may hear terms like zone blitzes, "robbers," and cloud or sky coverages. These are pro-style tactics with limited usefulness at the youth level. Essentially, a zone blitz is an exchange of responsibilities between a linebacker and a defensive lineman. As the linebacker blitzes, the lineman drops back into the linebacker's vacated zone. Ideally the quarterback will recognize that the linebacker is blitzing and try to throw the ball into his area, where the lineman will be waiting. I don't need to explain the disparity in talent between youth linebackers and youth defensive linemen for you to understand that this is a bad idea. A "robber" has some usefulness at the youth level, and I use a modified version of it for my cover two look in my 4-4 defense. Think of a robber as a zone blitz in reverse. The linebacker charges forward, and instead of a defensive lineman dropping into the vacated zone, a defensive back (usually the free safety) steps forward to cover the area. These techniques are effective largely because they change the basic zone landmarks, rather than because they confuse quarterbacks. (Most youth quarterbacks are already confused.)
The last two terms, cloud and sky coverages, are designations for specific types of zone coverages. Most zone pass defenses are landmark-based. That is, the pass defenders drop to specific areas of the field. Because of this, once you know that a team is in zone, it is not too difficult to design your passing plays to hit the areas between the zones. Sky coverages bolster a specific area of field by rotating all the pass defenders towards that zone. (This can be useful against the run.) Cloud coverages confuse landmarks by dropping the linebackers deep and using the corners to cover the short zones. Neither coverage is especially useful against youth football teams.