Mirror Mirror 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

This artifact enters tapped. , , Sacrifice this artifact: Choose target player. At the beginning of the next end step, exchange life totals with that player, exchange control of all permanents you and that player control, and exchange cards in your hands, cards in your libraries, and cards in your graveyards.