Natural or dry coffee processing is one of the oldest methods of processing coffees. Once the cherries are picked from the tress, they’re cleaned then spread out in the sun to dry on cement or raised tables. They’re left in the sun for weeks and are periodically raked and turned to ensure an even drying process and to prevent mildew growth. Because the coffee is laid out in the sun for weeks, natural processing is popular in regions that have very little rainfall, little access to water, and long periods of sunshine. Many coffees from Indonesia, Brazil, Yemen, and Ethiopia are dry processed.
As the cherries dry, they darken in color and harden. Once they’ve reach an appropriate dryness (between 10-12% for Specialty Grade) they are sent to a mill where they can be hulled, sorted, graded and bagged for shipment.
Coffees that have been processed naturally are often sweeter and have more body and a more complex flavor and aroma profile. This is most likely a cause of the fermentation process.
Believe it or not, the processing method (dry, wet, or pulped, etc) is one of the single most important factors in a coffee’s flavor, second only to picking appropriately-ripe cherries. There are still debates about which process is best and countries like Brazil are moving aggressively to modernize and automate as much as possible. What effects that modernization has on the flavor and quality of the coffee remains to be seen, but my concern is it will go the same route as other industrialized crops such as corn in the U.S.
Updated April 23, 2019: improved accuracy of some of the statements