Let's begin with some definitions:
Ok so an entity–relationship model or diagram (often shortened to just ER) is a data diagram used to describes a database in an abstract way.
When you look at the relational database, the schema defines several things:
- tables
- the fields in each table
- the relationships between fields and tables
It is this last point that makes accommodating ER diagrams into relationship schema so important.
Where the schema stores data in tables (some of which may point to
data in other tables), it is the relationship outlined in the ER diagram - that would say that a specific table is pointing to an entity, or has some other relationship with it
So a ER diagram is simply a diagram created to design these entities and
relationships.