In meteorology, precipitation is any product of the condensation of atmospheric water vapour that is deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel. It occurs when the atmosphere, a large gaseous solution, becomes saturated with water vapour and the water condenses, falling out of solution (i.e., precipitates). Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air. Precipitation forms via collision with other rain drops or ice crystals within a cloud.
For more, visit the Wikipedia entry for precipitation, URL: http://en.wikipedia.org/wiki/Precipitation_(meteorology)