This is the latest in the Zelda franchise. It is basically a remake of A Link to the Past. But it's got 3d graphics, and the ability to turn link into a painting to move along walls, and tons of new features and maybe even items. It's like an expanded version of ALttP, or maybe ALttP on steroids. By the look of the trailer, it's gonna be EPIC!
Oh, and in this version of the game, the Dark World is called Lorule. If that doesn't make sense, remember that the normal world is called Hyrule, which is pronounced as "high rule", so then Lorule is pronounced as "low rule". So indeed they are opposites, just like the light an dark worlds in the original ALttP. And Zelda's counterpart in Lorule (yep Zelda has a dark counterpart in this game) is called Hilda. Last 3 letters are the same, but there's more connection than that. Hilda and Zelda are both the names of witches in Sabrina The Teenage Witch. Wonder if there will also be a dark Link, as in OoT? Dark Link actually does appear, multiple times, called "Shadow Links". These are other players within wi-fi range of your 3DS handheld. Someone else's position in their game, appears as a Shadow Link in your game. And yes, you can fight them and defeat them to win prizes (like taking their rupee's, bombs, etc) which means that some other player who loses such a fight can actually end up losing stuff they've gathered in the game. A very interesting game mechanic. As for whether or not an actual Dark Link character as a CPU controlled boss or miniboss appears, I don't know. More info about the game will probably be forthcoming in the months up to its release.
Official site: http://zelda.com/link-between-worlds/