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 Righteousness.
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.