Cinnamomum zeylanicum is a native of Sri Lanka   and  Malabar   coast of India. Also grown   in Jamaica  and Brazil. But most of the world requirement of this spice is met by Sri Lanka hence it is known as Ceylon cinnamon or true cinnamon. An allied member of the same genes- Cinnamomum cassia(Cassia cinnamon), a native of China, Myanmar and Vietnam is often adulterated with true cinnamon. Other species like Cinnamomum burmani(Java cinnamon) Cinnamomum loureirii(Saigon cinnamon) are also equated with cinnamon.
Bark of cinnamon has wide application as a spice and  a flavouring agent. It has carminative, stomachic, astringent, anti allergic and antifungal property. Recent research throws light to the anti diabetic property of cinnamon. Water soluble cinnamon extract contain a chalcone polymer, MHCP, which   is  responsible for the anti diabetic activity. It stimulates insulin activity and enhance insulin signalling there by facilitates glucose uptake and helps to achieve favourable HBA1C.
True cinnamon bark is highly fragrant with smooth, dull, yellowish brown outer surface and dark yellowish inner surface. Bark is traded in the form of compound quills(one quill is put inside other) During drying  it curls from one side only and covers the other . This  cinnamon   sticks can be easily chewed or grinded.
Cassia cinnamon is darker in colour compared to true cinnamon and has stronger pungent to peppery taste with rough and uneven surfaces. They are curved inward from both sides towards the centre.
The major difference between true cinnamon and allied species is the toxic phytoconstituent coumarin content. True cinnamon has traces or undetectable level of coumarin where as cassia has high percentage of coumarin.
Coumarins are naturally occurring plant constituent made of fused benzene and a-pyrone rings. More than 1300 types of coumarins have been identified from plants, bacteria and fungi. Coumarins  are also found in plants like straw berry, lavender, liquorice, toncobeans etc and is also synthetically produced. Natural coumarins shows anti-inflammatory, antimicrobial, antihypertensive, antihyperglycemic  antiadipogenic, anti oxidant and neuroprotective  action.  
But used on regular basis, there will be cumulative toxicity. Studies shows that high level of coumarin are hepato toxic and   leads to liver damage.  Due to its appealing vanilla flavour coumarins are used as food additives and also in perfumery. In natural foods this   compound add distinctive taste and flavour and are extremely miniscule amount. But when people consume various cuisine containing cassia cinnamon   and other species or when used in powdered form, its limits exceeds and pose high risk.
Due to this toxic impact Federal Institute of Risk Management in Germany warned people to stop using   cassia.
True cinnamon has 0.001-0.005 milligram of coumarin per teaspoon. But in cassia cinnamon it is almost thousand times higher.
The tolerable daily intake is 0.1 mg per kg body weight. Coumarin is also found in tobacco and its use in cigarette is banned by tobacco companies in 1997. FDA has given coumarin the status as generally prohibited from direct addition or use as a human food.