Relic

Relic refers to a type of equipment worn in the relic equipment slot by death knights, druids, paladins and shamans. Relics give a significant bonus to a specific spell or ability, either as a passive equip or chance to proc effect (similar to trinkets of the same nature). Relics were added in Patch 1.10.0, replacing the previously vacant ranged weapon slot for these classes.

There were originally four different types of relics, each specific to a class:
 * Idols used by Druids.
 * Librams used by Paladins.
 * Totems used by Shamans.
 * Sigils used by Death knights.

These relic types were merged into one type (named Relic) in patch 4.0.1. Relics and the relic slot were removed in patch 5.0.4.

Cataclysm
In Cataclysm the relic system was simplified. Instead of four different types of relics, each one only usable by one class, all relics became usable by any class capable of using a relic. Previously existing relics were stripped of their unique equip effects, and were re-itemized to provide stats similar to a ranged weapon (primary and secondary stats). To compensate for the lack of weapon damage, each relic received a prismatic gem socket.

While legacy relic items still contained the words Idol, Libram, Totem, or Sigil in their names, they were not sub-categorized or restricted by those names.

Mists of Pandaria
Patch 5.0.4 removed the equip slot used for relics and ranged weapons, and removed relics from drop tables and vendors. All previously existing relics were converted into quality items.

Trivia

 * Prior to the introduction of relics, many druids, paladins, and shamans would often keep the quest item Egan's Blaster equipped, as this was the only item that could be equipped in the ranged slot for those classes.
 * This was also frequent during  Classic prior to phase 5, as relics were not available until then.
 * During  The Burning Crusade, Warlocks could equip spellstones and firestones in the same way (though in the ranged weapon slot as they can use wands).