Storm is a keyword ability on instants and sorceries that creates a copy of the spell for each spell played before it in the current turn.
The first cards with Storm were printed in Scourge, with more being printed in Time Spiral.
From the Comprehensive Rules:
- 702.39. Storm
- 702.39a Storm is a triggered ability that functions while the spell is on the stack. “Storm” means “When you play this spell, put a copy of it onto the stack for each other spell that was played before it this turn. If the spell has any targets, you may choose new targets for any number of the copies.”
- 702.39b If a spell has multiple instances of storm, each triggers separately.
- The storm copies are put directly onto the stack -- they aren't played. That means the copies don't generate storm copies themselves, and they aren't counted by other storm spells played later during the turn.
- Each storm spell with a target allows you to change the target for each copy of that spell. You make that choice for each copy individually.
- When counting spells played in a turn, you do count spells that were played face down, spells played from zones other than a hand, and spells that were countered.
- A copy of a spell can be countered, just like any other spell, but each copy has to be countered individually. Countering a storm spell won't counter the copies of it.
- Removing a card from the game using suspend doesn't count as playing a spell; you only play a suspended spell when you remove the last time counter from it and that ability resolves.
- When a spell like Twincast copies a spell that has storm, the copied spell's storm ability doesn't trigger. You get just one new spell.
- Grapeshot -
Grapeshot deals 1 damage to target creature or player.
Storm (When you cast this spell, copy it for each spell cast before it this turn. You may choose new targets for the copies.)
 Color percentages
There is a total of 19 cards that involve storm, which divide by color as such:
- Red = 42 %
- Green = 25 %
- Black = 11 %
- Blue = 11 %
- White = 11 %
 Notable cards with storm
 Storm scale
Mark Rosewater has declared that he doesn’t ever see Storm coming back. On Blogatog, he uses the"Storm scale" - a scale of 1 through 10 where 10 is Storm - when he answers questions about the likelyhood of the return of a mechanic.  1= will definitely see again, 10 = never say never, but this is pretty close to never.