A playing card was a small piece of card printed with symbols which was used in playing card games. Some were printed in the ultraviolet ranges, so they could be read, like Katha'sat, when they stopped at Starbase 18 (TOS novel: The Wounded Sky)
Playing cards could also be used to perform illusions, instructions for some of which could be found in the book 99 Great Card Tricks You Can Learn in Your Spare Time. (DS9 - The Lives of Dax short story: "Dead Man's Hand")
A popular card game was poker. In 2370 the crew of the runabout Amazon replicated a set of cards to play poker to pass the time on their 22 hour journey to the Davon system. (DS9 novel: Devil in the Sky)
Cribbage is another game, that you use them
Tongo was another card game.