Cinnamon

Cassia bark is an interesting spice. Also known as Chinese cinnamon, it is a genus of the cinnamon tree. Cinnamon is a little bit different from cassia, and usually differentiated by being called "true cinnamon." Cassia is cheaper to produce, and the majority of ground cinnamon is actually made from cassia bark. Indians use cassia instead of true cinnamon in their cooking, as it has a milder flavor and can be used in larger quantities.
Cassia can also be used whole or ground in spice mixes. It is easily distinguishable by its rough, tree bark-like texture, and the best way to check for freshness is to rub a little on your fingers. If you can smell a cinnamon fragrance, then the bark is fresh.
If substituting cinnamon for cassia, use less, as the flavor of true cinnamon is more intense.