"Worlds" are the playable factions in Future Card Buddyfight.
Comparable to Magic: the Gathering's colors and Cardfight Vanguard's clans, worlds divide the cards into groups based on mechanical and artistic themes, each World has its own artistic themes and playstyles as well as a unique card frame that represent the world's theme (Example: Dragon World is themed after dragonic creatures, is focused on destroying opponent monsters, and its card frame resemble fire)
Flag cards determine which World(s) you may use, because of that, it is recommended that your deck only includes cards from the Worlds listed in your Flag card (note that you can include cards from different Worlds, but they may not be used.).
Generic cards do not belong to any World, as such they can be used in any deck regardless of the World its focused on.