The big patch to the closed Hearthstone beta will be happening today. After you can log in at 6pm PDT (9pm EDT) you'll find that all of your cards have reset and everything is as if you've just logged onto the game for the first time. Except any money that you spent has been converted to in-game gold, which you can then spend on card packs and tournament entries.
This patch has been expected for awhile, and the card wipe itself has been known since day one of the Hearthstone announcement. Furthermore, Blizzard has said that they do not expect to perform any additional card wipes -- the cards and progress you make now will carry over to the live game upon release.
Blizzard's full announcement is as follows: