Condensed water vapor floating in air. They can take many different shapes due to wind patterns and moisture content. They play an important part in the world's weather because of the water they bring and because of their irradiative properties vis a vis global warming.
[Chemical and Physical Meteorology. v 41, 2, 1989.] [The Astrophysical Journal. v 333, 617, 1988.]
Source: Atmospheric Chemistry Glossary