In this article, the multiple roles of water in the Earth's atmosphere are reviewed. In particular, the behavior of atmospheric water in its various physical states such as water vapor, liquid water, and crystalline ice are discussed. Special emphasis is given towards its pure metastable and stable phases at ambient conditions. Moreover, the occurrence of these condensed phases in various types of atmospheric clouds is described. The thermodynamic boundary conditions for the formation of liquid-water clouds and of ice clouds are delineated, and the kinetics of the related phase transitions is discussed. Finally, a kinetic state diagram of atmospheric water is presented.