Magnetic Web 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

If a creature with a magnet counter on it attacks, all creatures with magnet counters on them attack if able. Whenever a creature with a magnet counter on it attacks, all creatures with magnet counters on them block that creature this turn if able. , : Put a magnet counter on target creature.