What part of the cinnamon is used?

Cinnamon is actually the inner bark of several different types of trees (in the genus Cinnamomum). It can be purchased as a cinnamon stick--the actual bark thinly cut, and dried. It can be purchased ground, where they take the stick/bark and process it down to a fine powder. It can also be purchased in an oil form, in which the oil is extracted from the bark and bottled to be sold.