Miss
Dodge
Parry
Glancing Blow*
Block
Critical hit
Crushing Blow†
Ordinary hit
This is the attack table. As far as tanks are concerned, combat in WoW is based on a /random 100 behind the scenes. You take your defense stats, line them up in that order (stopping when you hit 100%) and then random.
Some people don't get this, but it's important. If you have 5% miss and 5% dodge, the game doesn't roll twice, you can't have a swing be both a miss AND a dodge at the same time. This is why we can lump all the "zero damage" options, miss/dodge/parry, into one number and call that "Avoidance".
Crushing blows can only happen from a mob attacking a player, and only if the mob is 4 levels above the player. They do 150% of normal damage
Glancing blows are the opposite (sorta). They only happen when a player is attacking a mob, and according to wowwiki it's pretty consistently 24%.
If a number is 0, then you can skip right by it on the table. So if you're a player, you can never be glanced, so that's a 0 right there. Likewise, if the mob attacking you is 3 or fewer levels above you, there's no chance of crushes, so that's a 0 as well.
How is this important to tanking?
In BC, Crushing Blows could happen from bosses (boss = skull level mob = always 3 levels above the player for combat math) So we tanks did some finicky things to become "Crush Immune", that is, to push the Crushing Blows off the table, and our of possibility. Paladins had it easy, with holy shield being up almost all the time, and bosses attacking not very fast.
Forward to today, and the attack table is still important. Level 83 mobs (bosses) have about a 5.6% crit chance. The Defense skill, in addition to raising avoidance and block, also reduces the chance to be crit. This occurs at 540 defense skill (689+ def rating from gear). Tanks need to be crit immune, or they take crazy double-damage bursts, and when bosses hit for 20k+, double damage is instant death.
TLDR version: 540 defense skill is the absolute minimum stat you need to "Tank" raids.
