The cinnamon stick comes from the interior bark of a tropical evergreen tree. The cinnamon stick is the rolled up bark. There are two varieties of cinnamon, Cinnamon zeylanicum (Ceylon cinnamon) and Cinnamon cassia (cassia) with the Ceylon cinnamon being "true" cinnamon.
Cassia bark cinnamon is also referred to as "hard stick" cinnamon. Use this in beverages like mulled cider.
This is a pretty common grocery store item. Mostly you'll find the harder Cassia bark. Look for Ceylon cinnamon at grocery stores that feature Hispanic ingredients. You can also purchase Ceylon cinnamon online at Amazon.con