In lore, they have been attributed to be everything from cotton balls to candy. In reality, they are really much different.
Clouds are nothing but water in the air.
This water can take several forms, and it varies in formation according to height. Clouds under 6,500 feet are generally considered low, and are most often made of water. Clouds around 7,000 and 24,000 feet are considered mid-level clouds, and they are composed of mixtures of water and ice. Clouds above 20,000 feet are generally considered high-level clouds. These are the highest that clouds can go, and these are generally very wispy and long, made of ice.