Defense and Mitigation, how it all works.

Basic Concept:

The basic concept of tanking consists of three parts, maintaining the aggro, mitigating the damage and staying alive. There are three classes considered to be tanking class, the Warrior, the Paladin and the Druid. Though any class has some form to maintain aggro, mitigate damage and staying alive, only Warriors, Paladins and Druids have skills and talents specific for this purpose, as well as items made for them. This guide only talks about Warriors, Paladins and Druids.

Maintaining Aggro:

One aspect of tanking is making sure the mobs attack you, not the other people in your group, since you're made to take the hits where the rest is not. Mitigating your damage is useless if you're not the one getting the damage, and you'll more likely to survive a hit than the other people in your group.

The threat caused by Warriors and Druids scales with stats such as Strenth, Agility and Attack Power. The threat caused by Paladins scales with Spell Damage.

Mitigating Damage:

Another aspect of tanking is minimizing the damage you'll be receiving. It's completely useless to keep the aggro from the clothwearers, if you take as much damage as them. Your healers cannot keep you alive, unless you make sure to take the minimum amount of damage possible. Damage mitigation can be subdivided in two groups.

Damage Reduction means reducing the damage you'll receive when you get hit. The most basic form of damage reduction is Armor. The higher your armor value, the less damage you will take. A Warrior's Defensive Stance provides 10% Damage Reduction. A Paladin's Righteous Fury provides up to 6% Damage Reduction based on talents. These apply after each other. If you have a 60% Damage Reduction from Armor, then Defensive Stance only reduces 10% of the remaining 40% for a total of 64% Damage Reduction. Blessing of Sanctuary provides a flat Damage Reduction (80 at level 70), but before Damage Reduction from Armor and Stance is taken into account, so that Warrior with 64% Damage Reduction would only receive 28.8 less damage thanks to Blessing of Sanctuary. Block Value provices a flat Damage Reduction whenever you Block an attack, after Damage Reduction from Armor and Stance is taken into account.

Damage Avoidance means avoiding a source of damage altogether. Your Miss, Dodge and Parry chance avoid getting damaged altogether. Your Block chance avoids receiving Critical Hits and Crushing Blows, since these are mutually exclusive. Though avoiding getting damaged altogether is a nice thing, it's ony a chance, and therefor unreliable. It's chancebased, and might or might not happen. It won't do you any good if it doesn't happen. Don't go through great lengths just to boost complete avoidance. It's a little different for Critical Hits and Crushing Blows. With the right talents and gear, these can be avoided completely by Warriors and Paladins, making it something you can rely on. By reducing the chance of Critical Hits and Crushing Blows to zero, you cannot get unlucky.

Staying Alive:

The more Damage Mitigation you have, the less your healers will have to heal you, but you will receive damage. No matter how low the damage you take is, if the healers don't have time to heal you, you will die regardless. Your heath bar is there for the sole purpose to give your healers time to heal you back up before you take the next hits. It does not reduce the damage you take or the healing you need in any way.

If you completely strife for the most health, while disregarding actual damage mitigation, you will take more damage than your healers can keep up with, and you will die. If you completely strife for the most mitigation, while disregarding your health, an unlucky string of hits will kill you.

Damage Reduction from Armor:

Against mobs up to lvl59, the Damage Reduction in % is:
100 * Armor / (Armor + (Moblvl * 85) + 400)

Against mobs of lvl60 and up, the Damage Reduction in % is:
100 * Armor / (Armor + (Moblvl * 467.5) - 22170)

Against lvl73 mobs (Bosses are considered level 73 for level 73 player characters):
100 * Armor / (Armor + 11957.5)

The Damage Reduction for Armor is capped to 75%. Any Armor in excess of that becomes useless. For lvl73 mobs, this is 35872.5.

The Attack Table:

When a mob or character physically attacks another mob or character, WoW determines the result with a single roll. It adds all possible values together, using the following attack table:

Miss
Dodge
Parry
Block
Critical Hit
Crushing Blow (mobs only)
Regular Hit

There are formulas that determine the a character or mob's chance to Miss, Dodge, Parry, Block, Critical Hit and Crushing Blow in %. Those are placed below each other on a line from 0 to 100%. If the total of those values is less than 100%, the remainer is filled up with Regular Hit. If the total of those values is more than 100%, everything beyond 100% falls off the bottom (first Crushing Blow, then Critical Hit, then Block, etc...).

Example 1: There is a 10% chance a mob will Miss a character, 15% the character will Dodge his attack, 20% the character will Parry his attack, 15% the character wil Block his attack, 5% he will Critically Hit the character and 15% he will place a Crushing Blow on the character. Those values count up to 80%, so the remaining 20% becomes a Regular Hit.

One check is made. A result of 0-10% becomes a Miss. 10-25% becomes a Dodge. 25-45% becomes a Parry. 45-60% becomes a Block. 60-65% becomes a Critical Hit. 65-80% becomes a Crushing Blow. 80-100% becomes a Regular Hit.

Example 2: The character mentioned in Example 1 is a Paladin. He casts Holy Shield, which raises his chance to Block an attack by 30% (for a total Block chance of 45%). The values now count up to 110%, so 10% from the lowest value (10% Crushing Blow) falls off. A result of 45-90% becomes a Block. 90-95% becomes a Critical Hit. 95-100% becomes a Crushing Blow. There is no space for Regular Hits.

Example 3: The character mentioned in Example 1 is a Warrior. He casts Shield Block, which raises his chance to Block an attack by 75% (for a total Block chance of 90%). The values now count up to 155%, so 55% from the lowest values (all 15% Crushing Blow, all 5% Critical Hit, 35% Block) falls off. A result of 45-100% becomes a Block. There is no space for Critical Hits, Crushing Blows or Regular Hits.
http://wowvault.ign.com/View.php?view=Guides.Detail&id=200

The basic concept of tanking consists of three parts, maintaining the aggro, mitigating the damage and staying alive. There are three classes considered to be tanking class, the Warrior, the Paladin and the Druid. Though any class has some form to maintain aggro, mitigate damage and staying alive, only Warriors, Paladins and Druids have skills and talents specific for this purpose, as well as items made for them. This guide only talks about Warriors, Paladins and Druids.

The character mentioned in Example 1 is a Paladin. He casts Holy Shield, which raises his chance to Block an attack by 30% (for a total Block chance of 45%). The values now count up to 110%, so 10% from the lowest value (10% Crushing Blow) falls off. A result of 45-90% becomes a Block. 90-95% becomes a Critical Hit. 95-100% becomes a Crushing Blow. There is no space for Regular Hits.

There are formulas that determine the a character or mob's chance to Miss, Dodge, Parry, Block, Critical Hit and Crushing Blow in %. Those are placed below each other on a line from 0 to 100%. If the total of those values is less than 100%, the remainer is filled up with Regular Hit.

The more Damage Mitigation you have, the less your healers will have to heal you, but you will receive damage. No matter how low the damage you take is, if the healers don't have time to heal you, you will die regardless. Your heath bar is there for the sole purpose to give your healers time to heal you back up before you take the next hits. It does not reduce the damage you take or the healing you need in any way.

Damage Reduction means reducing the damage you'll receive when you get hit. The most basic form of damage reduction is Armor. The higher your armor value, the less damage you will take. A Warrior's Defensive Stance provides 10% Damage Reduction. A Paladin's Righteous Fury provides up to 6% Damage Reduction based on talents. These apply after each other. If you have a 60% Damage Reduction from Armor, then Defensive Stance only reduces 10% of the remaining 40% for a total of 64% Damage Reduction. Blessing of Sanctuary provides a flat Damage Reduction (80 at level 70), but before Damage Reduction from Armor and Stance is taken into account, so that Warrior with 64% Damage Reduction would only receive 28.8 less damage thanks to Blessing of Sanctuary.