Subtypes provide additional theme and flavor for a card. Subtypes are capitalized and shown in the card's typeline. For example, the card Roaming Monster has the 'Monster' subtype. In general, subtypes have no inherent game effect, but other cards may refer to them. For example, King of the Realm cares about the Mortal subtype. Artifacts have two special subtypes ('Automaton', 'Monument') that do have inherent game rules. See the those codex entries for more details.