Cinnamon is a spice obtained from the inner bark of several tree species from the genus Cinnamomum. Cinnamon is used mainly as an aromatic condiment and flavoring additive in a wide variety of cuisines.
The aroma and flavor of cinnamon derive from its essential oil and principal component, cinnamaldehyde, as well as numerous other constituents including eugenol.
Cinnamon is an evergreen tree characterized by oval-shaped leaves, thick bark, and berry fruit. When harvesting the spice, the bark and leaves are the primary parts of the plant used. Cinnamon is cultivated by growing the tree for two years, then coppicing it (cutting the stems at ground level). The following year, about a dozen new shoots form from the roots, replacing those that were cut.
The stems must be processed immediately after harvesting while the inner bark is still wet. The cut stems are processed by scraping off the outer bark, then beating the branch evenly with a hammer to loosen the inner bark, which is then pried off in long rolls.
Topically, both Cinnamon Bark Essential Oil and Cinnamon Leaf Essential Oil should be used with extreme caution, if at all. Tisserand and Young  indicate that both the bark and the leaf oil are low risk for mucous membrane irritation, may inhibit blood clotting, and pose a drug interaction hazard. Cinnamon Bark Oil may cause embryotoxicity and is contraindicated in pregnancy and breastfeeding. There is a high risk of skin sensitization with the bark oil.
You may see references to cinnamon essential oil as a "hot" oil. “Hot oils” are oils that can cause a hot or warming sensation when applied to the skin. They also have an increased incidence of dermal sensitivity and skin reactions and should be used under the guidance of a certified aromatherapist.
Robert Tisserand and Rodney Young, Essential Oil Safety (Second Edition. United Kingdom: Churchill Livingstone Elsevier, 2014), 248-250.