Clouds are made up of millions of tiny droplets of water and ice, or just water vapor, depending upon the temperature.
Each of these droplets of water is smaller than a grain of flour, and they are so light that they can float on air.
When pilots in airplanes fly through clouds they can see nothing but clouds. It's like being inside a thick fog or a steamy room.
There are 3 conditions that are necessary for cloud formation: lifting (evaporation), cooling and condensation.
We can prove that dry air absorbs water by evaporation. [link proof] Therefore it makes sense to say that the air around us contains water vapour (water vapour is a gas). When this air rises it gets cooler and the water vapour condenses (it turns into tiny drops of water) to form clouds. Did you realise that condensation is the reverse of evaporation?
Convection. Think about the saying that 'hot air rises'. When the sun's rays heat the earth this also heats the air just above the ground and this hot air rises.
Sometimes cold, heavy air pushes its way under warm air and forces it to rise.