The Madcap Jester is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever The Madcap Jester attacks, each opponent exiles the top card of their library. Whenever a spell or ability you control causes one or more cards to be exiled from an opponent's library, you may play those cards until the end of your next turn. Mana of any type can be spent to cast spells this way.