In a nutshell, air contains water molecules (or vapour). Warm air holds more water than cold air. Warm air rises and cools. As the air cools, the water molecules begin to condense (clump together) faster than they are torn apart by their thermal energy.
These clumps of water could not maintain form without a host of some sort, most typically dust. The condense water vapour clings to these airborne particles to form a droplet. It takes billions of these droplets to form a cloud.
Clouds are white because the condensed water vapour reflects the sum of the Sun’s spectrum, which is white. When the cloud is thick enough, the sun can no longer penetrate to the bottom (what we see from the ground) and thus the cloud grows dark and grey.
When it rains, the particles to which the water vapour attach are returned to the ground. Rainfall cleanses the sky by acting as a filter for these smaller airborne particles of matter.