If you look up on wikipedia you see the difference. They are all from the same genus Cinnamomum. The true cinnamon is Cinnamomum verum. The sticks are layered and brittle, while Cassia (also called Chinese Cinnamon) sticks are hard and may break your coffee grinder.
Anyway I find true cinnamon sticks in the delis 2 blocks away from me for a dollar or 2. There are many immigrants from Mexico, Guatemala, Ecuador etc. Only in the US and Canada Cassia is sold as Cinnamon.
Cinnamon - Wikipedia, the free encyclopedia
"Ceylon cinnamon, using only the thin inner bark, has a finer, less dense, and more crumbly texture, and is considered to be more aromatic and more subtle in flavor than cassia. Cassia has a much stronger (somewhat harsher) flavour than Ceylon cinnamon, is generally a medium to light reddish brown, hard and woody in texture, and thicker (2–3 mm (0.079–0.12 in) thick), as all of the layers of bark are used.[20]"
(...) Due to the presence of a moderately toxic component called coumarin, European health agencies have recently warned against consuming large amounts of cassia.[21] Coumarin is known to cause liver and kidney damage in high concentrations. Ceylon cinnamon has negligible amounts of coumarin.[22]"
Cinnamomum verum - Wikipedia, the free encyclopedia
Cinnamomum aromaticum - Wikipedia, the free encyclopedia
"Cassia cinnamon's flavour is less delicate than that of Ceylon cinnamon; for this reason, the less expensive cassia is sometimes called "bastard cinnamon".[2]
Whole branches and small trees are harvested for cassia bark, unlike the small shoots used in the production of cinnamon; this gives cassia bark a much thicker and rougher texture than that of true cinnamon (...)Due to a toxic component called coumarin, European health agencies have warned against consuming high amounts of cassia. (...)Cassia sticks can be distinguished from Ceylon cinnamon sticks in the following manner: cinnamon sticks have many thin layers and can easily be made into powder using a coffee or spice grinder, whereas cassia sticks are extremely hard, are usually made up of one thick layer, and can break an electric spice or coffee grinder if one attempts to grind them without first breaking them into very small pieces."