Snow, hail, sleet and rain are all forms of precipitation. Precipitation occurs when water vapor becomes too heavy to remain in a gaseous state and turns into a liquid (at warm temperatures) or a solid (at freezing temperatures). Solids and liquids are too heavy to float in the air like gases, so they fall to the ground.
Precipitation is an important step in the water cycle. Precipitation returns evaporated water to the Earth where it can be used by plants and animals. It is also important to keep too much water vapor from building up in the atmosphere. Water vapor traps heat on the Earth, and too much of it can cause the Earth to become too hot.