A storm is what happens when the earth's atmosphere is disturbed. If the weather report says that a storm is on the way, you might want to carry an umbrella — or just stay at home and watch movies.
Storms range from the windstorm that blows branches off your tree to the snowstorm that buries your neighborhood under two feet of the white stuff. It's a verb too: "It began to storm outside, and the rain beat down on the windows." Figuratively, you might even storm angrily: "My sister was so mad that she stormed around her room, kicking things." Something controversial can also cause a figurative storm, or uproar.