Breakdown monsters is a sub-type of Effect Monsters co-created by Carnage Infinity and LHK. They work mostly in groups of monsters and, like the " " series of cards, they have the evolution theme. However, the weakest monsters have the highest Levels, and the stronger the monster it is, the lower Level it has, but they can only be Breakdown Summoned. Breakdown monsters have a "Breakdown Level" (or Rank, for Breakdown Xyz Monsters) stated in their text, which indicates the Level of the monster of the same Type and Attribute as them that can be Breakdown Summoned from the hand once they are destroyed. The "Breakdown Level" (or Rank, for Breakdown Xyz Monsters) is a number that is lower than that of its actual Level. For instance, a DARK Dragon Breakdown monster with Level 8 and Breakdown Level of 5 that is destroyed can Special Summon a Level 5 DARK Dragon monster from the hand.
A thing to note is that Breakdown is not limited to Main Deck Effect Monsters, however, there are two differences; the Breakdown Summon applies while in the Extra Deck and that Breakdown Summon is treated as its corresponding proper Special Summon from the Extra Deck. For example, Fusion, Synchro and Xyz Breakdown Monsters follow the same procedure as above once destroyed, but Special Summon a monster of the same kind (e.g. a Level 8 DARK Dragon Synchro Breakdown Monster with a Breakdown Level of 6, once destroyed, can only Special Summon a Level 6 DARK Dragon Synchro Breakdown Monster from your Extra Deck, but it can't bring a Level 6 DARK Dragon Fusion Breakdown Monster from your Extra Deck that way), also they are considered as a proper Special Summon, which means that, once destroyed, they can be Special Summoned with " ".
Due to Link Monsters having no Levels, there are no Link Breakdown Monsters.