Cassia and cinnamon are almost identical (both belong to the same family of plants) but cassia is a lot more nutritious and tastes a lot different and better. Both, of course, are the wood, or bark of a plant, which is ground up. The nutrients are found in its intrinsic oils.
Dreaming of cassia or cinnamon speaks of a fragrance produced through crushing and suffering. It speaks of people and of course the church who, like Christ, are willing to suffer hardship and inconvenience to proclaim and preach the gospel of Christ.