The principle of an “Appellation d’Origine Contrôlée/Protégée” (“Controlled/Protected Designation of Origin”) is to protect a terroir and production expertise. They were created to combat fraud. The only difference between a PDO and an AOC is the scope of application.
Both designate a product which must be produced, processed and developed in a specific geographical area using recognised and recorded expertise.
For example: Bordeaux PDO protects Bordeaux wines across the entire European Union, whilst the AOC provides protection on a national level in French territory.