The Sun warms the sea, and some of the water lifts into the air as vapor, too fine to see. High up, where the air is cold, the vapor turns back into tiny drops, and the drops huddle together as a cloud. When the drops grow too heavy to float, they fall as rain. Rivers carry the rain back to the sea, and it all begins again.
Not to scale. The warmth slider stands in for the Sun's heat; the real Sun does not move closer. Real vapor is invisible and a real cloud is billions of drops. The cycle has no start and no end: the water you drank has been round this loop for billions of years.
USGS Water Science School: The water cycle
NASA Space Place: The water cycle
NOAA: Freshwater resources