The water cycle, also known as the hydrologic cycle, describes the continuous movement of water as it makes a circuit from the oceans to the atmosphere to the Earth and on again.
Most of Earth's water is in the oceans. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air. Rising vapor cools and condenses into clouds. Cloud particles grow and fall out of the sky as precipitation.
Some precipitation accumulates as ice caps, glaciers, and snowpacks; in warmer climates, snow melts in the spring and flows into streams. Most precipitation falls back into the oceans or onto land, where it flows over the ground as surface runoff. A portion of runoff enters rivers and continues toward the oceans. Runoff and groundwater seepage can also accumulate and get stored as freshwater in lakes.
Some runoff soaks into the ground where it replenishes groundwater aquifers or seeps back to the surface.
Over time, this cycle continues again and again.
Learn more: USGS Water Science School: The Water Cycle