This is a legit game, run by the actual Pokemon Company itself (in the US at AFAIK). And since it's run by an ACTUAL company, money will be involved to play. Oh no, not to sign up or anything. Signing up to the Pokemon website is free, and it's required for the Global Trade Station for Black and White, I think.
To play the game, you simply log in, make your avatar, and play online. The game will be divided in two modes:
In single player, you'll have three decks to choose from: a Fire, Grass, and Water starter deck, each focusing on the HeartGold SoulSilver series packs (not packs like HGSS Undaunted, Unleashed, etc.). There, you can challenge preset players with specialized decks. Take on the challenge and try to win all three championships.
If you've recently bought a Black and White Starter Deck (Emboar, Serperior, and/or Samurott), you may have gotten a card with a code on it. That code will allow you to use those decks on this game. Here is where I said money is required:
It's free to play, but to get cards, not so much. These code cards can be found in Starter Decks and Booster packs, starting from the HeartGold SoulSilver (there is a catch about the Booster packs, but I'll get to it later). With these cards, you can redeem the codes, and I ASSUME that you'll get a "Booster Credit." These Booster Credits will allow you to buy the pack in the game and add them to your online collection, as well as being able to use them in multiplayer games.
Now the catch is...you can only get these codes in box boosters, not Blisters. Blisters are the packs with the plastic covering and cardcoard-back. So be careful about that.
So yeah, give this a shot, and...if you guys have anymore info on this, contribute. Talk about it here.
Edit:
I also just found out that if you go to the second link and click "Collection", you'll have a deck already in there. Open it, and it's yours for the game. Beat the first challenges of Single Player with every deck and obtain it!