A cloud consists of many tightly grouped pinpoints. The pinpoints might be too small to distinguish individually at 10X, but together they look like wispy white or gray patches. Some clouds look like a light haze, while others are dense and almost impossible to see through.
Tiny clouds will not interfere with the flow of light, but large, dense, or numerous clouds can affect brilliance. Clouds are considered in the clarity grade, so the existence of clouds does not alter the grade, and only helps you identify the kind of inclusions that helped to determine the grade.