Cassia bark is related to cinnamon but it isn't the same. The two ingredients can be interchangeable but they do have a different emphasis on the flavour they infuse into your dish. Cassia adds a sweeter, milder flavour whereas cinnamon has a stronger more harsh, hotter flavour.
Cassia looks more like a thick outer bark of a tree and is darker in colour. Cinnamon is a lighter beige colour and made up of wafer thin quills. Cassia tends to be cheaper than cinnamon as it's cheaper to produce. For Indian dishes I will tend to use cassia rather then cinnamon.
It tends to be used in desserts as well as a number of savoury and rice dishes. Predominately used the Korma and Pasanda it gives the very subtle sweetness that these dishes are known for.
There is one variety called Cinnamomum which produces 'true cinnamon' most other cinnamon comes from related species that are also refereed to as cassia.