Wikipedia says that "the name cinnamon is correctly used to refer to Ceylon cinnamon, also known as "true cinnamon" (from the botanical name C. zeylanicum). However, the related species, Cassia (Cinnamomum aromaticum), Saigon Cinnamon (Cinnamomum loureiroi), and Cinnamomum burmannii are sometimes sold labeled as cinnamon, sometimes distinguished from true cinnamon as "Chinese cinnamon", "Vietnamese cinnamon", or "Indonesian cinnamon"; many websites, for example, describe their "cinnamon" as being cassia Ceylon cinnamon, using only the thin inner bark, has a finer, less dense, and more crumbly texture, and is considered to be less strong than cassia. Cassia has a much stronger (somewhat harsher) flavour than cinnamon, is generally a medium to light reddish brown, hard and woody in texture, and thicker (2?3 mm thick), as all of the layers of bark are used.
"... The two barks, when whole, are easily distinguished, and their microscopic characteristics are also quite distinct. Cinnamon sticks (or quills) have many thin layers and can easily be made into powder using a coffee or spice grinder, whereas cassia sticks are much harder. Indonesian cassia (Cinnamomum burmannii) is often sold in neat quills made up of one thick layer, capable of damaging a spice or coffee grinder."