Cards

From Elona Wiki

Cards and figures have a chance of being dropped by a monster upon death. When they are displayed in a museum, the museum attracts visitors and generates a profit. Museum rank increases by a smaller amount from cards than it does from figures, however. They are not sold in stores.

Cards also have the additional property of being able to be put into decks when they are used. Decks, however, are incredibly rare items, and as there is no way to remove cards from decks and no way of getting any use from cards in decks (currently), it is a better idea to simply save cards and put them into a museum.