User blog comment:ArtMaster7/Haliber the Artillery of Demacia/@comment-25403955-20141115072545/@comment-3542156-20141117123401

There are a few catagories of damage:

Spell damage, physical damage, magic damage and true damage.

Spell damage is all damage that is dealt by spells. Spell damage can be either physical damage, magic damage or true damage.

Non-spell damage can also be physical damage, magic damage or true damage. Sometimes even multiple of those three. (Corki with Statikk Shiv deals all three at the same time)

Armor reduces physical damage, regardless if it is spell damage or not.