Dryness is the most common physical feature of all deserts. Deserts can be mountainous or flat, and only about 20 percent of deserts are covered with sand. Some deserts are composed of rock expanses or salt flats.
Deserts are classified according to the reason for their dryness. The categories of deserts are subtropical deserts, coastal deserts, rain shadow deserts, interior deserts and polar deserts. Because of the arid climate of deserts, most desert plants are drought- or salt-tolerant.
Another feature of deserts is the lack of water. Most experts agree that the definition of a desert is a place where less than 10 inches of rain falls per year.