World Cards

World Cards are a type of map card in Kingdom Hearts: Chain of Memories which can transform floors of Castle Oblivion into entire worlds made out of Sora or Riku's memories. These cards are usually created by members of the Organization, but DiZ and Naminé are both able to make cards for Riku, and one appears on its own to King Mickey. Naminé is even able to fashion these cards into trinkets, as she does for the Riku Replica.

The World Cards are not all available at once. In "Sora's Story", Sora receives the Traverse Town card from Marluxia, the Wonderland, Olympus Coliseum, Agrabah, Halloween Town, and Monstro cards from Axel, the Atlantica, Neverland, Hollow Bastion, and 100 Acre Wood cards from Larxene, the Twilight Town card from Vexen, and the Destiny Islands card from the Riku Replica.

In "Reverse/Rebirth", Riku receives the Hollow Bastion card from DiZ, the Traverse Town, Agrabah, Monstro, and Neverland cards from Ansem, then the Atlantica, Halloween Town, Olympus Coliseum, and Wonderland cards from Ansem again, the Destiny Islands card from Zexion, the Twilight Town card from the King, and the Castle Oblivion card from DiZ.

In Kingdom Hearts coded and it's remake, Castle Oblivion is remade in data form, and the World Cards are still present, along with the Ending Cards.

Trivia

 * Although Sora, Donald, and Goofy visited the world Deep Jungle in Kingdom Hearts, it was not included in Chain of Memories as the estate of Edgar Rice Burroughs did not allow Disney and Square Enix to reuse the story. It appeared in early treatments for the game, though.
 * Though Castle Oblivion as a World Card is not obtained by Sora, and does not appear in his "World Cards" menu in Chain of Memories, it does appear in his "World Cards" menu in Re:Chain of Memories, and even uses the image of Riku's Castle Oblivion card as its depiction.
 * During cutscenes in Kingdom Hearts Re:Chain of Memories, the Destiny Islands card shows the Kingdom Hearts II design rather than the one used in Kingdom Hearts: Chain of Memories, however, once you actually receive the card, the design from the original game is shown to be on the card instead. The reason for this is unknown.
 * These cards may be the mechanism by which the Organization takes Sora, Donald, and Goofy's memories, and may have actually been created by Naminé. Except for in Traverse Town, Sora, Goofy, and Donald seem to have lost all their memories of each world they visit before they enter them, and the cards are said to be made by using their memories. However, the gang is able to at least recognize that they've been to each world, as they comment in Twilight Town that they don't recognize it.

Notes and References
Cartes Mondes