A Realm Reborn is the sub-title of re-released version of Final Fantasy XIV. It is also known as 2.0, consisting of game version numbers starting with 2. A Realm Reborn features completely rewritten code to the original version, better graphics, and a refreshed world to explore, as well as many additional features the original game didn't have.
You can read about differences between the two versions of the game here.
Trailers
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