To sum up Zeriyah's post, which can be found after the break it all its glory, the Arena is a place where you can build a pre-made deck, based around a choice of three of the nine available heroes. You'll then be given 30 sets of 3 cards, and pick one from each, making up a 30-card deck. There's no need to have any of the cards already, it could be anything. Winning rewards you with a key, and every time you win your key gets better, up to three losses. After three losses, that deck is done, and you start again. So if you win nine times, and then lose three times, you'll get the best possible rewards from your key.
This is a paid access area, so you have to pay to get in there with either real-world money or in-game gold, but as Zeriyah clarifies, if you win enough to get a Diamond key, you'll be back in for nothing. And yes, it used to be called the Forge! Hit the break for Zeriyah's full post.
Filed under: Hearthstone Insider