I believe you are referring to the unlockable cards. When you win 20 games vs. the AI with any faction, you "unlock" a special minion for that faction. You get one free copy of the card and can then purchase more copies (at the same price as buying an individual rare card, 200 gold).
Similarly, win 20 times with a faction in pvp to unlock a non-minion card.
In my opinion, the special minions in fire, jungle, and ocean are really good, the others are so so. The non minion cards in fire, swamp, air, and maybe underworld and forest are potentially useful, the others are rarely worthwhile except for special, rare circumstances.