Spirit Orbs are the primary method of obtaining new cards in Duelyst. Each Spirit Orb contains 5 cards. At least 1 card will be RARE or better. They can be purchased in the Armory or obtained as rewards from The Gauntlet or from Achievements.
To Open a Spirit Orb, click on the "Spirit Orbs" link in the lower left of the screen. You can view your acquired orbs in this screen, and only need to drag them to the center of the screen in order to open them.
Spirit Orbs from the Rise of the Bloodborn and Ancient Bonds sets operate a little differently in that they will give you three copies of three different cards from the set. However, once you have unlocked those cards, you will not receive additional copies of the cards as you might from other sets. Due to this, you can not disenchant or craft cards from these sets.