"Voodoo masks cloud the identities of umbaru witch doctors, presenting them in the aspect of the spirits. The masks confer protection, but also terrify foes and potential sacrifices with their inhuman visages."— Game Guide(src)
The Voodoo Masks are crafted from bone, wood, stone and feathers, rarely metal or even living beings, each mask being fashioned in an image of a spirit that the Witch Doctor wishes to take appearance of. In addition to protection of the head (and increasing the Witch Doctor's spell power), they also horrify the beholders with their twisted and otherworldly visage, instilling terror in the hearts of foes.
In addition to normal stats specific to Helms, Voodoo Masks also may roll Maximum Mana and Mana regeneration. Legendary Masks also often have unique affixes that greatly enhance the Witch Doctor's skills.
Voodoo Masks can be transmogrified to look like normal Helms, but only Witch Doctors can change normal helms into Voodoo Masks.