The "Extra Deck" is a special area in the Yu-Gi-Oh! trading card game where players can store specific types of cards that are not part of their main deck. This includes Fusion Monsters, Synchro Monsters, Xyz Monsters, and Link Monsters. Players can summon these cards during their turn by using certain game mechanics, allowing for advanced strategies and powerful plays. Cards in the Extra Deck are typically more powerful than regular cards and often require specific conditions to be summoned. Players can have a maximum of 15 cards in their Extra Deck, which adds an element of strategy in choosing which cards to include for their gameplay.