Premium cards (look sort of like foil cards, I guess) show up every once and a while when you buy a card pack. And there is a "Premium Room" card where you can increase your chances of getting premiums. They are also listed separately in the deck manager. Why should I care about them? The stats looks the same, but are they for some reason better?
Premium cards cost fewer points to put in your deck compared to the regular version, so in theory you use them to build a bigger deck. However, the problem is premium cards ALSO don't get reshuffled after use (just like the first card used in a sleight.). Because of that, I never liked them much. Your play style may vary.