Mirror of Twilight

The Twilight Mirror is the link between Hyrule and the Twilight Realm. Made by the gods, the mirror allowed people to travel in between the two interlinked worlds as pleased. Located in the desert, the Twilight Mirror was used to banish and imprison the most evil of criminals of the kingdom into the Twilight Realm, never to be seen again.

The location of the Mirror of Twilight was well known by most of Hyrule, but many believed it was just a myth. When Link reached its resting ground, the Arbiter's Grounds, he was forced to fight a massive beast, the huge Stallord, in order to reach what remained of the Mirror of Twilight. The Mirror is found within the Mirror Chamber, and is composed of two parts:
 * The Mirror itself, which acts as a kind of 'projector' which activates the portal.
 * The stone where the portal forms. The Mirror shines onto this enormous stone slab, creating the portal to the Twilight Realm.



It is then revealed by the sages that Zant used his newly acquired powers from Ganondorf to break the mirror into four shards, but didn't have enough power to shatter it completely (which Midna succeeds in doing at the very end of the game). Link and Midna must then find all of the shards to complete the Mirror of Twilight and transport them to the Palace of Twilight.

Despite the fact that the mirror is utterly destroyed by Midna, there may be other ways of traveling between the two worlds as Midna mentions many times that she and Link may meet again.

Trivia
The Mirror of Twilight, although officially debuting in Twilight Princess, may possibly have appeared in Ocarina of Time as the large round mirror in the Spirit Temple that granted Link access to the chamber of the temple's boss, Twinrova. Considering that mirror's role in Link's quest to rid the temple of Twinrova, it's similar appearance and the temple's many similarities to the Arbiter's Grounds, the Mirror of Twilight may very well have appeared even sooner in the series than previously thought.