Final answer:
An ERD includes the relationships, table structure, and primary keys, which help visualise the database schema. Primary keys provide unique identifiers, relationships show connections and dependencies between tables, and the table structure outlines entities, attributes, and constraints.
Step-by-step explanation:
The ERD, or Entity-Relationship Diagram, covers three essential pieces of information when depicting the structure of a database. These are relationships, table structure, and primary keys. An ERD is a visual representation that allows designers to conceptualize how data is connected within a system, offering a clear view of the database schema.
Primary keys are unique identifiers for each table that ensure each record can be uniquely identified. Relationships depict how tables in the database are connected to each other, showing the cardinality and dependency between entities. The table structure consists of the entities represented as tables, their attributes, and the constraints applied to them. Although ERDs may contain information about foreign keys and data types, these are not considered as the core essentials.