What is the storm ability in Magic the Gathering?

In the card-based game Magic the Gathering, to "storm" means to duplicate the spell being cast for each previous spell in the same turn in the card stack. This ability is triggered when the card is on the stack.

New targets may be chosen for any of the resulting copies of spells. There are a number of stipulations that include the following:

• The number of copies is set when the storm spell is played.
• The copies affect the spells in the stack, but they are not played.
• Multiple instances of storm within a spell will trigger independently of each other.
• Each copy of the spell must be countered separately; countering the storm spell itself does not counter the copies.