Deserts are areas where rainfall is very low. In many desserts few plants can grow but other deserts support specialised plant and animal life. There are few mammals in deserts and desert mammals tend to be small. Cold blooded reptiles and invertebrates are more frequent in deserts.   
We usually think of deserts as very hot, very dry and covered in sand but this isn't always true. Some deserts are cold, others are rocky and/or mountainous. Examples of cold deserts include Antarctica and the Gobi Desert in China.