Spirit world

The spirit world (also called ghost world, astral realm, or world of spirits ) is the place where creatures go when they die, it could be described as the generic term of "afterlife" in the Warcraft universe.

Background
The final destination of the spirits of the deceased remains a mystery to the priests and philosophers of Azeroth. However, as spells such as resurrection can reunite a dead body with its spirit, and a majority of living creatures from the tauren to the troll shadow hunters claim they can communicate with and call upon the power of the spirits, a widely held belief is that the spirits of the dead remain on the Material Plane — in an immaterial state that can only be altered or contacted through the use of magic.

World of Warcraft
In World of Warcraft it can be accessed in Azeroth and Outland when a character die. They can be resurrected with an Spirit Healer or by finding the corpse of their physical body, though this may be just a game mechanic.

The spirit world can also be accessed by other means, such as with some specific Reagent Pouch. It can also be seen with some devices and potions while alive.

World of Warcraft: Wrath of the Lich King
The Lich King gained rule over the spirit world with the help of the val'kyr.

Description
In World of Warcraft, the spirit world is identical to the Material Plane, with the only difference that in the spirit world the character can only be seen by other spirits.

Traits
While in the spirit world, visions of past events may be seen.

Sometimes, the spirits roam the spirit world without resting. It can be because their resting place wasn't complete, because of a battle in their resting place or because the corruption of their resting place. To bring eternal peace to the souls, it is needed to destroy the source of corruption or by making a ceremony with Tuskarr Ritual Object.

Day of the Dead
During the Day of the Dead, the spirit world draws closer to Azeroth.

Speculation
Given how the Spirit world seemingly is connected to the dead, it may be related to or even be the same as the Realm of Shadows which is connected to the undead.