Deserts are places without water - or at least, without very much water. But they're not always hot. In the United States, the Great Basin desert, for example, is a 'cold' desert because its mean annual temperatures are low.
Deserts can also be classified on the basis of their rainfall. If a desert has less than 12 centimeters of rain in a year, its called a 'true desert'. If it has more than 15 but less than 40 centimeters per year, its called a 'semi-desert'.
