Power Pack 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

Flying, vigilance, trample, haste Whenever Power Pack deals combat damage to a player, exile target instant or sorcery card from your graveyard chosen at random. At the beginning of your next upkeep, you may cast that card without paying its mana cost. If that spell would be put into your graveyard, exile it instead.