In the US, cinnamon and cassia are used interchangeably. True cinnamon (cinnamomum verum) is dried into thin cylindrical quills. Cinnamon has a sweet perfume, warm and woodsy with no trace of bitterness. Cassia (cinnamomum cassia) is thicker, coarser bark found in flat pieces (smooth dark brown inner bark and corky rough gray brown outer bark) and thicker reddish-brown color quills. Ground cassia is intense, sweet, and lingering. Typically, cassia is preferred in the US because of its more pronounced aroma and flavor. Used around the world in both sweet and savory dishes.
Our Indonesian and Vietnamese cinnamons are cassias and our Sri Lanka cinnamon is true cinnamon.