Damage types and how much armor protects against them:

Acid -- Protects strongly
Asphyxiation -- Armor has no effect
Cold -- Protects moderately
Disintegration -- Protects somewhat
Electrical -- Protects very little
Fire -- Protects moderately
Harm -- Protects very little
Holy -- Armor has no effect
Illusion -- Armor has no effect
Physical -- Protects very strongly
Poison -- Protects very little
Psionic-- Armor has no effect
Sonic -- Armor has no effect
Unholy -- Armor has no effect

See also: HELP ARMOR PROTECTION AGAINST DAMAGE TYPES