Windmill Slam 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

Create a 4/4 Windmill artifact creature token with defender. When you do, it deals damage equal to its power to any target. Miracle (You may cast this card for its miracle cost when you draw it if it's the first card you drew this turn.)