I started with creating a card game for iOS and I'm interested whether or not could I create multiplayer over GameCenter or only over WiFi?
Game Center is an Apple-provided interface for connecting players. Wi-fi is an internet connection method. You can use Wi-Fi to connect to other players anywhere in the world, through Game Center.
If you were looking for information on how to create a game with Game Center, I would check out http://www.raywenderlich.com/3276/how-to-make-a-simple-multiplayer-game-with-game-center-tutorial-part-12.
If you need information on how to create a multiplayer card game, I would look at http://www.raywenderlich.com/14618/how-to-make-a-simple-playing-card-game-with-multiplayer-and-bluetooth-intro-and-challenge