Defenders make an invaluable part of any adventuring party. Many monsters have quite nasty attacks, and when monsters are free to use those attacks to their full effect against a vulnerable controller, striker or leader, an encounter can quickly turn into a defeat for the player characters. Defenders make the whole party's job easier by limiting monsters' ability to do just this, and thus enable their allies to act unhindered.
The prototypical tank defends by balancing their defenses against a punishing mark. This poses a dilemma for the marked foe - do they attack the defender, and thus waste their attacks on the most durable opponent, or do they ignore the mark, attacking who they had planned to attack, but at the cost of a -2 penalty to hit and quite possibly an additional form of punishment, such as a fighter's combat challenge attack?
A good defender thus combines their high defenses with their mark to thwart their enemies' attacks, regardless of the choice their enemy ultimately makes. This is known as the lose/lose dilemma of defending. If a marked enemy attacks the defender, they lose, and if they attack somebody else, they lose. This is an important point worth stressing - if a defender fails to make both horns of the dilemma sufficiently unappealing, then the defender cannot fulfil their role effectively. For example, a level 1 defender with AC 14, 23 hit points, and no defensive tricks to speak of will be crushed by anything they mark, and because they are probably the weakest member of the party anyway, using the mark will have achieved nothing (as it encourages enemies to do what was probably in their own interests anyway!)
It is important to note right from the outset that when I use the term "punishment" I usually mean it in a generic fashion, covering any outcome that is unfavorable for the enemy and thus can be used as a punishment or deterrence for a given course of action. Different defenders have different means of "punishing" enemies for attacking their allies. Fighters, Paladins and Assault Swordmages do it primarily by dealing damage. Shielding Swordmages punish enemies by reducing the damage they deal. Wardens and Ensnaring Swordmages are more likely to punish enemies with forced movement and making them more vulnerable to future attacks. The two key features of punishment are that it is conditional upon the target taking a particular action and that it is undesirable to them and thus gives them a reason not to take that action.
Because marks from multiple defenders do not stack (they overwrite each other), two defenders usually cannot effectively both "tank" a given enemy at the same time. However, there are a number of ways that a second defender can synergise beautifully with the other party's defender, and the purpose of this guide is to illustrate but a few of the ways that two defenders can work together well.
As are all guides, this is very much a work in progress. As I don't have encyclopaedic knowledge of every class, I'd be eternally grateful of any feedback and suggestions - particularly any interesting second-defender powers or builds that I've neglected to mention. I will have missed most of them while the guide is still new :D