The Extra Deck (エクストラデッキ, Ekusutoradekki; previously referred to as Fusion Deck, 融合デッキ, Yūgō Dekki) is a Deck where Fusion Monsters, Synchro Monsters, and Xyz Monsters are stored during a Duel. They are separate from the Main Deck, and you can search through it to find a Fusion Monster, Synchro Monster, or Xyz Monster that you need. There is a 15-card limit to the Extra Deck, regardless of how many Fusion Monsters, Synchro Monsters, or Xyz Monsters you include. The "Maximum of 3 Rule" for cards of the same name still applies.
If a Fusion Monster, Synchro Monster, or Xyz Monster that is on the field, in the Graveyard, or banished is returned to its owner's hand or Deck, it is instead sent to the Extra Deck, since Fusion Monsters, Synchro Monsters, and Xyz Monsters cannot exist in either the hand or the Main Deck.
The contents of your Extra Deck are not public knowledge to your opponent, and only you may look through its contents. During a Duel, the Extra Deck is placed face-down.
Any card or card effects that still refer to the Fusion Deck now apply to the Extra Deck.
Limits on the amount of cards that may be stored in the Extra Deck in the anime and manga have yet to be mentioned.