What is rain? (in English)

Rain is liquid precipitation, as opposed to other kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface. On Earth, it is the condensation of atmospheric water vapor into drops of water heavy enough to fall, often making it to the surface. Two processes, possibly acting together, can lead to air becoming saturated leading to rainfall: cooling the air or adding water vapour to the air. Virga is precipitation that begins falling to the earth but evaporates before reaching the surface; it is one of the ways air can become saturated. Precipitation forms via collision with other rain drops or ice crystals within a cloud. Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops.

詳細的資料:http://en.wikipedia.org/wiki/Rain

To put it in a simple way...

When the sun evoporates the water from sea/ocean, the vapor will caught up (process called, evaporation) in the atmosphere. When enough volume of vapors being gathered, a cloud will be formed. Gradually, when the cloud becomes heavy (because of way too many vapors being carried), the vapors will drop back to the Earth's surface. This process is called, condensation.