When the warm air, laden with moisture, rises into the sky, it cools down into water vapour. The excess moisture changes into small drops of water or bits of ice and these form clouds.
Since clouds always change their shapes and are formed at different heights and temperatures, no two clouds are alike. The highest clouds are called Noctilucent clouds. They may be made of ice.
Below Clouds info from Wikipedia (Read More):-
A cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae. Clouds are studied in the nephology or cloud physics branch of meteorology.
http://en.wikipedia.org/wiki/Clouds
