Choice cards are special uncollectible cards which appear when choosing between possible effects. In most cases, Choice cards only exist during the selection process and cannot normally be collected or placed in the hand or deck. Choice cards were originally exclusive to druid cards with Choose One effects, but are now available to all classes through a range of neutral cards.
Prior to Patch 188.8.131.52933, most choice cards used the same card art as either the card that produced them or a card that they themselves produce. With the addition of Keeper Stalladris, all Choose One spells were given unique art to more easily tell them apart in your hand. Most minions still use the art of the card they produce, however.
Note that this list may not show choice cards from the Core set, since their choice cards are a part of their original set, not the Core set.