When you are looking for celery seeds, you will see that some seed companies include celery root in the mix even though the main event is not the stalks and the leaves. All three types have a celery flavor.
Stalk or Stem: This type is what you typically find at the grocery. The main attraction is the juicy thick stems.
Chinese or Leaf: This type has thin hollow stems with a stronger flavor. Most folks use the leaves and the stems for seasoning like in soup.
Celeriac or Root: While its formal name is Celeriac, the more common name is celery root. This type is know for the root which grows into a ball under the ground. Once it is peeled, it can be eaten raw. I like to shred it on a salad. You can also cook with it.
