Read this from wowpedia: http://wowpedia.org/Chaos_(spell_school)
Chaos refers to a type of magic wielded by warlocks. Chaos is a unique multi-school, combining all the seven schools of magic: Arcane, Fire, Frost, Holy, Nature, Shadow and even Physical. Multi-school abilities benefit from bonuses that affect any of their schools, use the lowest resistance value among their schools, ignore absorption and immunity effects that apply to only some of their schools, and can be used even when one or more of their schools have been locked with an interrupt effect.
Chaos' comprehensive nature guarantees that damage done will not be mitigated by any armor, resistances (such as Shadow Resistance) or school-specific effects (such as [Twilight Ward]) the target might possess. However mitigation and immunity abilities that are not school-specific (such as [Defensive Stance], [ice Block] and [Divine Shield]) as well as damage-absorption shields provide by abilities like [blood Shield], [illuminated Healing] and [Power Word: Shield] will still absorb, mitigate or completely prevent damage from this school. Due to its comprehensive nature, it is impossible for players to be locked out of the Chaos school.
Chaos spells
Spells that use Chaos include Demonology warlock abilities available while in [Metamorphosis]:
[Chaos Wave]
[Touch of Chaos]