A cloud is a clump of water vapor that you can see in the sky. Clouds are sometimes white and puffy, and other times so big and gray that they completely block the sun.
Clouds form when air rises and cools and the moisture in it condenses into a visible form. Any similar billowy mass is also a cloud, like a cloud of smoke over your campfire or a cloud of dust after your car peels out of the driveway. If you're gloomy or morose, someone might say there's a cloud over you — a figurative cloud of sadness. On the other hand, if you're "on cloud nine," you couldn't be happier.