Forbidden Status only exists in Advanced Format, since Traditional Format does not possess a Forbidden List. When a card is Forbidden, no cards of that name can be used in your Main Deck, Side Deck or Fusion Deck.
Japan also uses the Forbidden List.
As of March 1st, 2007, these cards are Forbidden in Advanced Format:
Forbidden List 「禁止カード」
Black Luster Soldier - Envoy of the Beginning
Breaker the Magical Warrior
Butterfly Dagger - Elma
Change of Heart
Chaos Emperor Dragon - Envoy of the End
Chaos Sorcerer
Cyber Jar
Cyber-Stein
Dark Hole
Delinquent Duo
Exchange of the Spirit
Fiber Jar
Graceful Charity
Harpie's Feather Duster
Imperial Order
Last Turn
Last Will
Magical Scientist
Magician of Faith
Makyura the Destructor
Mirage of Nightmare
Monster Reborn
Painful Choice
Pot of Greed
Raigeki
Sinister Serpent
The Forceful Sentry
Thousand-Eyes Restrict
Time Seal
Tribe-Infecting Virus
Tsukuyomi
Victory Dragon
Witch of the Black Forest
Yata-Garasu
In Japan, the following are also Forbidden:
- Temple of the Kings 「王家の神殿」
- Sixth Sense 「第六感」