Cassia (Cinnamomum cassia) is a sibling from cinnamon (Cinnamomum zeylanicum) and is sometimes called Chinese cinnamon. The cassia is a tree that is endemic in southern parts of China, but is now in cultivation in many countries in Southeast Asia. The tree grows to a height of about 15 meters with elongated leaves that have a reddish color when they have just opened up on the tree. In summer it blooms with yellow flowers. Since time immemorial, even the flower buds are used as a spice in countries such as India, a habit that was emulated by the Romans.
The first part of the scientific name, Cinnamomum, obviously has the same origins as the English name for cinnamon. Because cinnamon has been traded by Arab merchants since time immemorial, the word cinnamon also has a long history: the Ancient Greeks borrowed their word kinnamomum from the old Hebrew word kinamom and the Aramese qunimom. At the end of its journey it may even be connected to the modern Indonesian word kaya manis (‘sweet wood’). The second part, cassia, can be traced back via the Greek word kasia (κασια), which is derived from the Hebrew words quetsi'oth, qase'ah or variations thereupon. It has its roots in the word qēș which in turn is derived from the Semitic qșș. It means 'to cut' and explains the 'harvesting' of the cassia bark.
Cassia has a warm, aromatic and slightly bitter taste. Compared to cinnamon, its better known brother, it is slightly more bitter. That does certainly not mean that cassia is less tasty, but that it will give a slightly different taste sensation when used in dishes.
This spice is packed with essential oils, which consists mainly of cinnamic aldehyde. Recently it has become clear that cassia also contains considerable amounts of coumarin and that is a so-called allergen. Coumarin, along with a number of related compounds, is responsible for the typical smell of drying hay and dried grass. During the drying of these plants these substances are released from the sugar chains to which it is bound in the living plants. Coumarin is a precursor of the anti-coagulant dicoumarol and may cause ingestion, liver damage, kidney damage and headaches.
After reading the above, you shouldn't conclude too hastily that you do not want to add cassia to your food. Like all spices, it should be used sparingly. Too much is excess and will ruin every single dish. Used in small quantities, there's no danger whatsoever, but it will lead to delicious curries. One can replace cinnamon with cassia in every recipe. And vice versa.
See also: https://www.naturalspices.com/ground-cinnamon.html