Location
They are found in the interiors of continents, away from coasts. This is because the warm moist air comes off the sea and causes rain in coastal areas, so by the time the air has got to the interior, there isn't any moisture to cause rain any more. They are found in a belt along 30 degrees north and south of the equator.
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Soil
Lack of soil. If there is any then it is infertile. Some hot deserts contain salt flats
Water
Some water sources in the form of oases made from aquifers deep under ground. (You don't need to know how they are formed) Less than 250 mm of rainfall every year.
Temperature
Temperatures are hot in the day and near freezing at night. It is hot in the day because the lack of cloud cover means that the land is exposed to the full heat of the sun. It is cold at night because the lack of clouds mean that they can't act as a blanket to keep the heat from the day in. Therefore the heat from the day escapes very quickly.
Nutrient cycle and biodiversity
The lack of biodiversity is caused by a very slow nutrient cycle which is due to the hot weather and lack of rainfall. If there is a slow nutrient cycle then the soils of the desert will be infertile. Infertile soils are not good enough to support large amounts of biodiversity.