Deserts cover more than one-fifth of the Earth's land area, and they are found on every continent. A place that receives less than 10 inches (25 centimeters) of rain per year is considered a desert. Deserts are part of a wider class of regions called drylands. The desert biome is an ecosystem that forms due to the low level of rainfall it receives each year. Deserts cover about 20% of the Earth. There are four major types of desert in this biome - hot and dry, semiarid, coastal, and cold. They are all able to inhabit plant and animal life that are able to survive there. A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation.