Almost one-third of Earth’s surface is desert -- that is, area that gets less than 10 inches of rain each year. These areas have what’s called a moisture deficit: they lose more moisture through evaporation than they receive. Only about 20 percent of Earth is sand, however. Most deserts on earth are located at about 25 to 30 degrees north and south of the equator. The amount of desert on earth is increasing by approximately 40 square miles each day, in a process known as desertification. Some of this is natural, and some results from human acts such as slash and burn agriculture or overgrazing.
A true desert is both dry and has limited plant and animal life. Although people tend to think of deserts as very hot sand dunes a la "Lawrence of Arabia", deserts can be cold, or rocky. The percentage of true desert has increased in the last 125 years, from about 9% to 14%. According to the Gale School link cited below,
True deserts cover about 14 percent of the world's land area, or about 8,000,000 square miles (20,800,000 square kilometers). Another 15 percent of the Earth's land surface possesses some desertlike characteristics. Most deserts lie near the tropic of Cancer and the tropic of Capricorn, two lines of latitude lying about 25 degrees from the equator.
About a tenth of the Earth's surface is covered by deserts.
A desert is defined as an area with less than 250 mm (10") of rainfall a year. This includes about 10% of the Earth's total surface.