Durability is a property of equippable items that determines how much damage they can take before being destroyed. Durability damage is not the same as normal damage.
All armor slots take durability loss excluding: Neck, Tabard, Back, Finger, and Trinket slots.
Repairing items
Any NPC who sells weapons or armor (or Blacksmithing supplies) can repair durability:
- As of Patch 2.3, your pointer changes when you move it over a repair-capable NPC:
interact with repairing vendor 
repairing vendor out of range
- Repair all equipped items (button should look like an anvil with a gold +).
- Repair a single item (button should look like a hammer).
Item repair is not currently an available skill for Player Characters, but engineers can craft repair bots.
Repair Cost
The base cost to repair an uncommon armor piece (no weapons or shields) assuming you are neutral with the vendor is:
Durability points to be repaired * (ilevel - 32.5) * 0.02 silver
The base cost to repair a rare armor piece (no weapons or shields) assuming you are neutral with the vendor is:
Durability points to be repaired * (ilevel - 32.5) * 0.025 silver
The base cost to repair an epic armor piece (no weapons or shields) assuming you are neutral with the vendor is:
Durability points to be repaired * (ilevel - 32.5) * 0.05 silver
The reputation discounts are taken from the prices above as follows: 5% for friendly, 10% for honored, 15% for revered and 20% for exalted.
What reduces durability?
The following situations can reduce an item's durability:
- Being in combat at all will cause all equipped items to gradually lose durability, regardless of whether any damage is taken or dealt.
- There is a small chance with each hit by your weapon that it will lose 1 durability point. (Parry? --Fandyllic)
- There is a small chance with each spell cast that you will lose 1 durability point to your weapon.
- There is a small chance with each hit absorbed by your armor that it will lose 1 durability point. (Shield block also? --Fandyllic)
- When you die, 10% durability is lost on all equipped items (that have a durability rating). This is 10% of maximum durability, not remaining durability. You will not lose 10% durability on death if you are killed in PvP combat by another player (if a PvP flagged guard or other NPC kills you, you still lose durability) or if your death is suicide through using an item or spell that kills you (this excludes environmental damage, such as falling).
- When you use the services of a Spirit Healer, you lose 25% durability on all equipped items and items in your bags, on top of the 10% durability loss on your equipped items you already lost when you died.
What happens when durability gets low?
From Blizzard: "Items remain at full effectiveness until they have no durability. Their power does not degrade."
When the durability on a specific item drops below 25%, a small stick figure (looks like an armored soldier) appears below the Mini-Map and the corresponding part of the stick figure (head, shoulders, chest, etc.) appears in yellow.
When the durability on a specific item reaches 0%, the corresponding part of the stick figure (head, shoulders, chest, etc.) appears in red. Once the durability of the item reaches 0%, the player can still wear or wield the item, but he does not receive any stat benefits (stats, resistances or armor). If a weapon breaks though, you fight as if unarmed. Items cannot permanently break!
Note: if the broken item is a Set Item, the set bonuses will still be active. ru:Прочность