* Déserts: This is the plural form of "désert," which means "desert" (as in a dry, barren area of land).
* Desserts: This is the plural form of "dessert," which means "dessert" (as in a sweet course served after a meal).
So, if you are talking about dry, barren areas of land, you would spell it déserts. If you are talking about sweet treats, you would spell it desserts.