A group of cards usually with 1 defining Attribute or Type, and typically have one or two words in the card name signifying that Archetype. Examples of an Archetype include: Archfiends, Elemental HERO, Ancient Gears and the various LV Monsters. Others may include monsters without the certain word in their name, for example, the Gladiator Beast's Test Tiger and the Lightsworn's Judgment Dragon.
A list of Fan Made Archetypes can be found Here