Shield

A shield is a type of armor carried in the off-hand equipment slot. A shield may also be referred to as an aegis or buckler.

A shield contributes to the Armor and can prevent damage by blocking attacks. Warriors, Paladins, and Shamans are the only classes that may use shields. A shield cannot be used with weapons that require two hands to wield, as shields fill the off-hand slot. However, Warriors with Titan's Grip may use a 2-hander and a shield. Paladins and warriors have several abilities can only be used when a shield is equipped, such as Spell Reflection and Shield of the Righteous.

Shields have far more armor than any other piece of equipment. A shield has roughly 60% as much armor as a full set of plate armor of the same item level.

There are two broad types of shields: those with defensive statistics like defense rating or dodge rating, suitable for tanking warriors and paladins, and those with spellpower, suitable for holy paladins and elemental and restoration shamans.

Shields have one major difference from other armor pieces; they can be equipped or removed during combat. All other armor pieces must remain equipped/unequipped until the Warrior/Paladin/Shaman is no longer in combat.

Bucklers
During the early beta days of World of Warcraft there were two shield types: normal shields and bucklers, which were lighter and could be equipped by shamans, rogues and hunters. Patch 0.8 turned all bucklers into shields and gave shamans the ability to use shields but rogues and hunters could no longer use them.