Cards are collectibles in Gabrielle's Ghostly Groove 3D. When you go to the Witch's Hut, Crystal will read your fortune based on the card you pick. These cards are based on the Major Arcana-cards from the Tarot deck, but the pictures show Gabrielle and her friends.
There are 22 cards total. In order to collect them all, simply visit Crystal multiple times. There's no limit to how many times you can visit per day.