While the sun is evaporating water, it is also heating up pockets of air as well. Just like a hot air balloon, these pockets of warm, moist air begin to rise into the atmosphere taking water with them. Sometimes these small pockets will mix with other pockets to make larger air masses. As the air masses rise, they also begin to cool. Clouds come in varying shapes and sizes depending upon how they were formed.
Most clouds are in a constant state of creation and destruction.
The water vapor inside them condenses in one spot, only to be re-evaporated in another. Sometimes, a cloud is formed very quickly from a mass of very warm, moist air. When this occurs the cloud will become thick and black with condensing water droplets. Once back on the Earth, the cycle can start all over again. The inside of a cloud is actually quite exciting.