An Entity Relationship Diagram (ERD) or Entity Relationship Model is a graphical representation of entities and their relationships to each other, typically used in computing in regard to the organization of data within databases or information systems.
ERDs can be used to depict two different kinds of models that are closely related, but different in intent. One kind is an analysis model of the subject matter. This model breaks the subject matter down into entities and relationships among entities. Subject matter entities are just subject matter objects by a different name, except that in data analysis, we aren't trying to model the behavior of these objects or entities. Values to be stored in the database can be grouped into attributes, and attributes can be connected to entities and relationships that the attributes describe. This is the ER model as originally developed, and the intent is to describe the problem, not the solution.
Another kind of ERDs is a depiction of the table design of a relational model. This relational model will be eventually fleshed out with physical details, finally resulting in a construction plan for the database itself. While table design is very closely related to ER modeling, it isn't exactly the same thing. for example, normalization is relevant to table design, but not to requirements analysis.
While an ERD can be used for either one of these two purposes, it's useful to decide which of the two purposes one is pursuing before starting in on a diagram.