Karlov's Crossbow 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 equipped creature attacks, sacrifice Karlov's Crossbow and destroy target creature an opponent controls. Equip Forecast — , Reveal Karlov's Crossbow from your hand: For the remainder of the game, spells you cast named Karlov's Crossbow cost less to cast. (Activate only during your upkeep and only once each turn.)