Tornado 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

Cumulative upkeep (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.) , Pay 3 life for each velocity counter on this enchantment: Destroy target permanent and put a velocity counter on this enchantment. Activate only once each turn.