Niv-Mizzet's Diary 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

Imprint - Whenever a player cards an instant or sorcery spell, exile it as it resolves. At the beginning of your upkeep, you may copy an imprinted card and cast the copy without paying its mana cost. It can't cause you to take extra turns. Then, if there are three or more cards imprinted on this artifact, sacrifice it and create a token that's a copy of Niv-Mizzet, Firemind.