Manual

Armor Sets
Rumors persist of unique, distinguished item sets that once belonged to great
heroes and legends. These sets are said to bestow additional magic abilities once
all their pieces have been found and equipped. When a character is equipped with
all the items of a set, additional bonus magic attributes are added as well. When a
set item is selected, the names of all the items in the set are listed on the tool tip.
Durability
As weapons and armor get used, they start to wear down. Eventually, armor that
continues to get battered in combat will begin to degrade in quality, as will
weapons that keep biting into the tough hide of monsters.
In World of Warcraft, every time you use a weapon or suffer a blow in battle, your
weapon or armor has a chance to take a hit in durability.
Your equipment's current durability can be seen by mousing over it from your
Character Info window or your inventory. When a piece of equipment drops to low
durability, you will see a small representation of your character and the damaged
equipment in the corner of your screen.
When a piece of equipment's durability gets very low, you can repair it at a
merchant NPC who specializes in working on the particular item that requires
attention.
If an item drops to zero durability, it become useless and will no longer
confer any benefits to your character. It will remain equipped on your
character, but armor will cease to provide protection and weapons will be so worn
down that you will effectively attack as if you had no weapon.