A Schema is a set of rules for how entities, attributes, and relationships between entities can be arranged in a database. While similar to knowledge graph ontologies, database schemas typically refer to rules on how traditional databases work. Historically, the primary difference between a schema and an ontology is that a schema provides rules centered around ordering data, while an ontology provides rules centered around ordering relationships. The focus on relationships within ontologies tends to reduce complexity in modeling meaning (making data semantic).
See also, ontology.