From Diablo Wiki
Hearthstone is a Warcraft-themed digital card-dueling game created by Blizzard Entertainment. Heartstone is a free to play computer game, though buying additional cards is not free, and is essential for success at the highest level. Hearthstone entered beta testing in 2013.
The official trailer:
 Hearthstone Official FAQ
From the official site:
What is Hearthstone?
Hearthstone is a free-to-play digital strategy card game that anyone can enjoy. Players choose one of nine epic Warcraft® heroes to play as, and then take turns playing cards from their customizable decks to cast potent spells, use heroic weapons or abilities, or summon powerful characters to crush their opponent.
On which platforms will Hearthstone be available?
Hearthstone will initially be available for Windows® and Macintosh®, with a version for iPad coming later.
What are the system requirements for Hearthstone?
We’ll announce specific system requirements closer to launch.
Do I need to know anything about strategy card games or Warcraft to play?
Not at all! We’ve designed Hearthstone so that anyone can get into it, no matter how familiar they are with the Warcraft universe or collectible card games. The game will teach you everything you need to know as you play it.
How do I apply for the beta test?
Just opt-in with your Battle.net account!
How will I get cards in the beta test? Do I need to purchase them with real money?
We plan to test the in-game store during the beta test, and players will be able to purchase card packs using their Battle.net Balance or other Battle.net-supported payment methods. Players will also be able to obtain cards and card packs free of charge through Practice mode, Play mode, and Arena.
The card pack price in the beta test does not necessarily reflect what the final prices will be at launch.
Will the cards I earn or purchase in the beta test carry over into the live game? What happens if I purchased beta card packs for real money?
During the course of the Hearthstone closed beta, card collections and game progress will be reset, and any cards participants acquired will be removed. Following the wipe, players who made a purchase from the in-game store will receive a gold credit equivalent to their previous card pack and/or Arena purchases. Also, as a thank you for testing out the in-game store during Hearthstone’s beta phase, everyone who purchased a card pack in the beta test will receive a Golden Legendary card.
Will I be able to add friends in Hearthstone and chat with people?
Yes! The folks in your Battle.net friends list (both BattleTag and Real ID friends) are available for games or text-based conversation. Players will not be able to chat with anyone who is not on their friends list. However, players will be able to choose from a variety of emotes for basic communication during games.
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