Clouds represent Perlin noise. In addition, each noise-based Blender texture (with the exception of Voronoi and simple noise) has a Noise Basis setting that allows the user to select which algorithm is used to generate the texture. This is often used for Clouds, Fire, Smoke. Well-suited to be used as a Bump map, giving an overall irregularity to the material.
The standard noise, gives an intensity.
The noise gives an RGB value.
Soft or Hard, changes contrast and sharpness.
The dimension of the Noise table.
The depth of the Clouds calculation. A higher number results in a long calculation time, but also in finer details.