The Twilight World is the Realm to which the Ancient Twili were banished.
They lived there in peace until Ganon managed to contact a wizard of the Twili named Zant. Zant then took his hordes to conquer Hyrule from the Twilight World with new powers obtained from Ganon. In the end, Ganon uses Zant to free himself. I'd say its a spoiler, but lets face it, everyone saw this coming.
__________________
|