In North America, the most common spice labeled as cinnamon is actually cassia, also known as Chinese or Saigon cinnamon. It is harvested from the bark of the evergreen Cinnamomum aromaticum tree. Cassia is cheaper to produce and has a bolder, less subtle flavor than true cinnamon.
The one concern is that cassia contains coumarin, which in taken in large quantities can worsen liver disease in those with that condition.