When a priest casts the barkskin spell upon a creature, its skin becomes as tough as bark, increasing its base Armor Class to AC 6, plus 1 AC for every four levels of the priest: Armor Class 5 at 4th level, Armor Class 4 at 8th, and so on. This spell does not function in combination with normal armor or any magical protection. In addition, saving throw rolls vs. all attack forms except magic gain a +1 bonus. This spell can be placed on the caster or on any other creature he touches.
In addition to his holy symbol, the caster must have a handful of bark from an oak as the material component for the spell.
Notes: Common for druids (PHB).