Strictly speaking, clouds are made out of water vapor, when there is enough water clinging to specks of dust in the air to be visible to the eye. By the time it gets to be much more than wet air, it turns into rain
At least, that’s the way it works before magic gets involved, because there are plenty of wizards who want to build their castles in the clouds, and plenty of air elementals who would like a place to put their more solid possessions.
Cloudstuff looks like a cloud, but beyond the layer of vapor coating the surface feels more like thick sponge: more than solid enough to support an object placed upon it. Slicing cloudstuff open (hardness 3, 5 hp per inch of thickness) reveals more of the same – although pieces smaller than one cubic inch dissolve into simple water vapor.
Cloudstuff is lighter than air at sea-level, and if left unattended will float up to roughly 8,000 feet in altitude, where the air is slightly thinner. One cubic foot of cloudstuff effectively weighs -1 pound, and if weighted down with that much weight becomes neutrally buoyant in air, remaining wherever it was put, even mid-air.
While typical cloudstuff is the same white as clouds on planets like Earth, through combinations of water density, dust particles and non-water vapor the clouds can be any color, and even patterned given enough delicate work.
Cloudstuff is worth 10 gp per cubic foot, but rarely is it bought or sold in volumes less than dozens of cubic feet at once.