Heat from the sun causes water on Earth (in Oceans, Lakes etc) to evaporate ( turn from liquid to gas) and rise into the sky.
The gas would then turn into clouds. After a while the clouds would then produce some water wich would then come flying back to the ground this is called precipitation.
When the rain hits the ground and goes into the ground it is called infultration.
Erosion is when the water hits the ground and rubs the dirt and then goes further.