The hydrologic, or water cycle, is one of the most important systems on this planet because it replenishes water.
The water cycle starts when water from the oceans and lakes is evaporated by the sun. This water goes up into the air in the form of clouds. After the water cools down and condensates in the upper atmosphere, it starts falling back down to earth.
The water can be in different forms when it comes down, like snow, ice, or rain, but all of this precipitation eventually ends up in the same place. It either soaks into the ground or evaporates again.
The water that soaks into the ground is called the groundwater and usually appears out of the ground in springs, which become rivers, which go to the sea, starting the cycle all over again.
Sometimes, groundwater is used up by plants. The water in the plants then evaporates back into the air in a process called transpiration.
On very tall mountains, you can find snow all year round, because it is very cold there. Sometimes, this snow goes straight to gas form without passing through liquid state, a process called sublimation.