Spirit Realm

Spirit Realm in Diablo 3 can refer to one of two things related to the Witch Doctor.

First of all, the Witch Doctor has a variety of Spirit Realm Spells. These Skills are focused more towards Crowd Control and Ranged abilities that can deal Physical Damage. As most of the Witch Doctor's Skills do, Spirit Realm Spells require Mana to cast.

The Spirit Realm is also a place the Witch Doctor can enter by using the spell Spirit Walk. While in the Spirit Realm, the Witch Doctor's movement is unhindered and cannot be seen by enemies. This ability is essential for Witch Doctor's who cannot take a lot of damage, especially in the arena.

Also See:
Movement Abilities

Note: After taking fatal damage the Witch Doctor Passive Skill Spirit Vessel automatically makes the Witch Doctor enter the Spirit Realm for a few seconds.