Often, deserts are the result of a rain shadow. Prevailing winds hit a mountain range, and the air cools as it moves up over the peaks. That cooling results in rain and/or snow, which removes most of the moisture from the air.

As the moving air gets warmer as it moves down out of the mountains. That warm, dry air means that the area downwind of the mountain range gets very little rain. It also means that any moisture evaporates very quickly.

Not all deserts form this way, but if you look on the east side of major mountain ranges around the world, you will find quite a few deserts.