The flavour of cassia is aromatic and mildly hot.
It is the inner bark of a tropical evergreen tree and is usually harvested as a single strip of bark and rolled into a stick. This stick is noticeably harder than cinnamon.
Cinnamon and cassia are very similar. Outside of India or Asia little distinction is given, though cassia tends to dominate the market. Cassia is also known as Chinese cinnamon, which is the most common commercial item you will find.
By appearnce, cassia is much darker than cinnamon. The rolled bark will hard a single hard layer, whilst cinnamon will have several softer layers.
We use cinnamon in desserts or sweeter dishes. More robust dishes and masalas use cassia.
Cassia is available from specialist spice merchants and good Indian grocers.