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 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.
In many supermarkets in the United States (and in Australia of course), products labelled as cinnamon may often be cassia. Due to the presence of a moderately toxic component called coumarin, European health agencies have recently warned against consuming high amounts of cassia. This is contained in much lower dosages in Cinnamomum burmannii due to its low essential oil content. Coumarin is known to cause liver and kidney damage in high concentrations. True Ceylon cinnamon has negligible amounts of coumarin.
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. Saigon cassia (Cinnamomum loureiroi) and Chinese cassia (Cinnamomum aromaticum) are always sold as broken pieces of thick bark, as the bark is not supple enough to be rolled into quills.
It is a bit harder to tell powdered cinnamon from powdered cassia. When powdered bark is treated with tincture of iodine (a test for starch), little effect is visible in the case of pure cinnamon of good quality, but when cassia is present, a deep-blue tint is produced, the intensity of the coloration depending on the proportion of cassia.
Cinnamon is also sometimes confused with Malabathrum (Cinnamomum tamala) and Saigon cinnamon (Cinnamomum loureiroi).