Cinnamon sticks are the rolled bark of the evergreen tree from which cinnamon is made. Ceylon Cinnamon sticks (the sticks are also called quills) will have many thin layers and can be ground in coffee grinders to produce a fine powder. Cassia Cinnamon sticks are made up of one harder layer and do not grind well.
Cinnamon sticks are used in mulled or steeped drinks where the stick sits in liquid. Some flavor and aroma is extracted into the drink using this method. This can take a number of days (especially steeping in the refrigerator).
The sticks are also used as a garnish and to stir drinks.