A schema diagram is a visual representation of the structure of a database. It illustrates how different tables are related to each other, showing the organization of data and the relationships between various entities. This helps database designers and developers understand how data is stored and accessed. In a schema diagram, each table is typically represented as a box, with lines connecting them to indicate relationships. Common elements include primary keys, which uniquely identify records, and foreign keys, which link tables together. Schema diagrams are essential for effective database design and management, aiding in the development of systems like SQL databases.