Direct Elemental Damage (e.g. "+15 Elemental Damage") is dealt 1/3 Fire, 1/3 Lightning and 1/3 Cold Damage (in the example: "5 Fire Damage, 5 Lightning Damage, 5 Cold Damage"). Each of the damage types is resisted by its corresponding resistance.
Percent Elemental Damage (e.g. "+10% Elemental Damage") is applied to each of the elemental damage types (in the example: "+10% Fire Damage, +10% Lightning Damage, +10% Cold Damage") and also increase potency of all Elemental Damage over Time effects.
Armor has no effect on Elemental damage, only resistances help against it.