Celery grows in a collective unit of ribs that grow together, joined at a common base. This unit is referred to as a head of celery.
Although stalk and head are both considered to refer to the collection of celery ribs, many individuals confuse stalks with ribs. A rib of celery is an independent piece from the head or stalk, but it's important to clarify terminology on a recipe calling for a stalk. The intent may be to use an entire head, or it may be to use just a rib, and clarification may be need from the resource in which a recipe is published.