Although Cinnamon is obtained from the bark of several trees from the genus Cinnamomum, it is only the one called Ceylon cinnamon derived from the plant Cinnamomun verum (real or true cinnamon) or Cinnamomum zeylanicum, that is beneficial (especially when taken for medicinal proterties). It has a low Coumarin content (a chemical compound that is moderately toxic to the liver and kidneys) unlike the Cassia. It also has a taste that is sweet and delicate (the Cassia one is strong and peppery).
Cassia is a member of the same family as Cinnamon referred to as "Chinese cinnamon", "Java cinnamon",
"Padang cassia" or "Saigon cinnamon”. It is cheaper to produce and often mistaken for, and even misnamed and sold as cinnamon by traders.
However Cassia and Cinnamon are not the same.
Look out for softer, smoother, paler, tan-cloured, tightly rolled (single direction roll like a newspaper) quills to identify real cinnamon as different from Cinnamon Cassia (which is the bark of the Cassia tree) that is harder, rougher, thicker, reddish-brown or dark-brown, and usually curl inwards from both sides.
Wikipedia link to Cinnamon.