There are a couple of main ways we play defence. Here is the start of a quickie reference.
Types of Defence
Defences used at Sunday Football fall into 2 major categories: Man and Zone. Man is simplest, but requires players to match up well physically and skillfully with the offensive player they are covering. Zone defence is more complex and can be picked apart by a smart quarterback.
Less common is the Ma-zone, but that's more of a retroactive call.
Man, or "man-to-man", is a conceptually simple defence. Each player picks (or is assigned) a corresponding receiver from the offensive team. Each defender is responsible for covering that player for the entire play.
Generally speaking, it is used for simplicity (for defenders not comfortable with zone) or when there's a favourable match-up between the defenders and the receivers (i.e. the defenders think they're better than the corresponding receivers). Difficulty arises when the receivers are better/faster/stronger/taller/skiller than the defender, or when lots of crossing patterns are used.
Quick Switch/Quick Cross
A common variation happens when there are 2 or more receivers lined up on the same side of the QB. One of the receivers' defenders may call "Quick cross", "Give them the quick cross", or "sides". This means that if the receivers run a short crossing play (before reaching the defenders) then the defender should cover whichever receiver comes toward them, rather than the one one they lined up on. From that point on, the defender then remains covering that receiver.
There are a variety of zone defences, but the general principles remain. Zone is often used when the defenders do not match up well against the receivers, or against certain quarterbacks, like Blake.
Unlike Man-to-man defence, the defenders will often look at the QB to see where he's looking to anticipate the throw, or to help choose which receiver to cheat to. A skilled quarterback will intentionally look away from their intended target to "look off" the defender, before turning to their desired receiver and throwing.
The general principle of the zone is that each defender is responsible for a "zone" or area of the field and covers any receiver entering their zone but leaves them if they leave the area for another defenders zone. This can be effective against crossing patterns as the defenders simply wait to see who ends up in their zone and don't get caught up in the crossing. However, this requires communication by the defenders to ensure that the receivers are "handed off" correctly. Typically a defender whose receiver is leaving their zone for another's calls "crossing" and awaits the other defender calling some acknowledgment, like "got him". If more than 1 receiver enters their zone, then a neighbouring defender is expected to follow the 2nd receiver into that zone, usually from the zone where the receiver just was. This is also hard to co-ordinate and is a good technique for QBs to defeat the zone. Often called "overloading the zone".
The zone layouts vary and that's where some of the difficulty comes in. There are some general principles to follow, and some common layouts that can be combined into more complicated zones.
When the zone calls are a series of numbers (e.g. 2-1, 1-2-1, 3-2), it means how many defenders are at each "level", starting from the front (closest to the line of scrimmage).
This is a basic building block of zones, as it can represent 1 "layer", and also it is not an uncommon zone call in itself. In it each defender is responsible for a "swim lane" that extends from the line of scrimmage all the way to the goal line. Any receiver that comes into their zone is their responsibility. Essentially the rightmost zone defender is responsible for the rightmost receiver, and similarly for the leftmost defender. Thus, if all the receivers go to one side, the defender of the zone they go away from must come in to cover the receiver closest to their side.
Initially the defenders line up approximately splitting the field into equal zones, but cheating a little to line up approximately on a receiver. If the receivers are bunched up on one side, they shift to be closer to those positions, but keep the receivers "inside" of them i.e. the defenders on the outside zones are closer to their nearer sideline than the receiver is.
Example #1: Normal
In the diagrammed case, usually called "3 across":
A: starts to move to the right with receiver 1, but when he sees (or "B" calls) receiver 2 crossing into his zone, he stops and follows 2.
B: starts to move left with receiver 2, but when he sees (or "A" calls) receiver 1 crossing into his zone, he stops and follows 1.
C: Since 3 moved outside and there are no more zones to that side, C follows receiver 3.
General note: once the receiver is behind the defenders position, no more switching occurs. So if receiver 2 doubled back into the middle zone, defender A would continue to cover him, as B is still covering 1.
Example #2: Stacked
In this case, with the receivers all on one side, the zone shifts to the left with them, but defender C is still closer to the right sideline than receiver 3, and A is closer to the left than 1.
A: As before.
B: As before.
C: Since no-one comes to his zone, follows the rightmost receiver (3) to the left, into B's zone. Since B has followed 1 deep, C stays with 3 all the way.
2 defenders in the "front" or short zone, 1 defender deep. Deep defender covers the middle. Short zones cover their sides, leaving deep middle patterns to the deep defener, but following any deep patterns that go outside e.g. Flag/Corner, out-and-up.
The 2 short zone defenders line up with the receivers inside them, about 5 yards off the line. The deep zone defender lines up in the middle, approximately on the QB, 10-15 yards off the line.
A: Follows 1 all the way as he goes deep to the outside
B: When he sees (or C calls) receiver 3 doing a post or an in-and-down or other deep middle play, calls defender C off and covers 3.
C: Initially covers 3 but as he sees 3 go deep and hears B call him, lets him go and shifts back to cover 2 as he crosses under him, and then follows him deep as he goes down the outside.