Clouds form through the evaporation and condensation of water. As water molecules shift between vapor, liquid and solid, they move through the air. When a small amount of air cools very quickly and reaches saturation, the water can condense and become a cloud.
Several factors can create a cloud. For example, the terrain can push water molecules upwards into cooler air, known as orographic lifting, or the vapor could enter a cold front. Another cause of clouds forming is the water vapor clinging onto something. When water vapor clings to something, it can change into its liquid or solid state. Water vapor can cling to many particles, which are called condensation nuclei or freezing nuclei.
Dust particles, soot from wildfires and sea salt particles can serve as condensation nuclei, and the water droplets or ice crystals will begin to form around them, get cold quickly and become clouds. Even certain plant bacteria can be the focal point for cloud formation.
A cloud is essentially just a large amount of crystallized water molecules and water droplets. The different shapes, sizes and other features of a cloud all depend on the conditions in which they were developed. Factors that affect cloud formation include humidity, temperature and altitude.