"Utopia", known as "King of Wishes, Hope" (希望皇ホープ Kibō'ō Hōpu) in Japan, is an archetype associated with the "Number" archetype. It is a sub-archetype to the "Hope" archetype. They are used by Yuma Tsukumo and Astral as their signature cards in the anime, and are supported by cards in the "ZW -" archetype.
"Utopian Aura" and "Utopia Buster" are not "Utopia" cards because they do not have 「希望皇ホープ」 in their Japanese names.
Aside from "Number 39: Utopia" itself, the monsters have effects which either increase their own ATK or deal with the ATK of the opponent's monsters.
A Utopia Deck usually focuses on Xyz Summoning "Utopia" monsters. The base form negates attacks, and the Number C39 ,Rank-Down, and Rank-Up versions focus on gaining ATK and draining opposing monsters' ATK. The alternate "Utopia" forms can be easy summoned by most "Rank-Up-Magic" cards and Rank-Down-Magic Numeron Fall. This archetype works well with "Number Wall", "[[Onomatopia" and the "ZW -" archetype.
Two great Xyz Monsters that can support this archetype are "Number 99: Utopic Dragon and "CXyz Barian Hope". The former can be easily summoned to the field by simply discarding a "Rank-Up-Magic" card and using a "Utopia" monster as Xyz Material. "Barian Hope" can be Xyz Summoned by "Rank-Up-Magic Astral Force". It is capable of using the effects of "Utopia" monsters in the Graveyard.