The Extra Deck (previously referred to as "Fusion Deck") is a Deck where Fusion Monsters and Synchro Monsters are stored during a Duel. They are separate from the Main Deck, and you can search through it to find a Fusion Monster or Synchro Monster that you need. There is a 15-card limit to the Extra Deck, regardless of how many Fusion Monsters and/or Synchro Monsters you include. The "Maximum of 3 Rule" for cards of the same name still applies.
If a Fusion, Synchro, Xyz, or Link Monsters on the field, in the Graveyard or Removed from Play is returned to its owner's hand or Deck, it is instead sent to the Extra Deck, since Fusion Monsters and Synchro 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.