Short answers, pulled from the story.
A desert is a region of land that is very dry because it receives low amounts of precipitation and often has little plant cover. Most deserts receive less than 250 mm of precipitation each year. Semi-deserts, or steppes when grassed, receive between 250 and 500 mm.
Antarctica is the world's largest cold desert, composed of about 98% continental ice sheet and 2% barren rock. It qualifies as a desert because little precipitation falls there, with the main form being snow rather than rain. About one-third of Earth's land surface is arid or semi-arid.
The Atacama is blocked from precipitation by the Andes mountains to the east and the Chilean Coast Range to the west, while the cold Humboldt Current and the Pacific anticyclone keep its climate dry. Some weather stations there have never received rain, and evidence suggests the Atacama may have had no significant rainfall from 1570 to 1971.
Desert plants such as cacti store water in their trunks, reduce or abandon their leaves, and grow shallow or deep root systems, while many complete their life cycle in weeks after rain. Animals adapted to deserts are called xerocoles, and many never drink, drawing water from their food and the water of metabolism. The camel can lose 40% of its body weight to dehydration without dying.
Deserts are classified by geographical location and dominant weather as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts. Trade wind deserts like the Sahara occur at 30 to 35 degrees north and south, while the Taklamakan is a rain shadow desert receiving less than 38 mm of precipitation a year. In 1961 Peveril Meigs sorted desert regions into extremely arid, arid, and semiarid categories by rainfall.
Desert nomads such as the Tuareg ran caravans across the Sahara carrying slaves, ivory, gold, and salt, and several million slaves may have crossed the Sahara between the 8th and 18th centuries. Desert farming relies on irrigation, as in California's Imperial Valley, which draws Colorado River water through the All-American Canal. Deserts are also used for solar energy, with plants like the Ivanpah Solar Power Facility built in the Mojave Desert.