Rain occurs because of the condensation of water in clouds.
When water evaporates from the earth, it collects in the atmosphere into clouds. When the water in clouds condenses to the point of liquifying, it rains.
The water on earth goes through a cycle when it first exists as vapor in the atmosphere, then, when weather conditions are favorable, it condensates and form clouds which then in turn "break" apart in the form of rain drops. The liquid water on earth flows in rivers, forms lakes and oceans and eventually becomes either frozen in the ice of the poles or evaporates to form more clouds.
picture taken from: www.louisianacoastalwetlands.com