Joker in cards is the same as a wild symbol in slots; however, initially this word meant 'jester', while nowadays we often remember Joker as the main antagonist of the Dark Knight movie about Batman. Anyway, jokers have been connected to cards and gambling since long ago, hence the creation of slots with the joker theme.
Even if you aren't that experienced in online slots, you probably know card symbols are featured in nearly all video slots out there, and some of the classic slots, too. However, those cards are usually the AKQJ letters, which means ace, king, queen, and jack; no joker!
Still, slots that do feature jokers are many as well, and most likely those jokers are going to serve as wild symbols, as this is what actually jokers do in regular card decks: they substitute any cards! Jokers may well be used as scatter or bonus symbols, too, as well as just normal symbols with high payout.
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