The ER diagram represents entities, attributes, and relationships. An entity is a representation of a real-world object such as car or a user. An attribute is a property of an object and describes it. A relationship represents an association between two or more entities.
The attributes might be composite or simple (atomic). Composite attributes can be divided into smaller subparts. A subpart of a composite attribute provides incomplete information that is semantically not useful by itself. For example, the address is composed of street name, building number, and postal code. Any one of them isn't useful alone without its counterparts.
Attributes could also be single-valued or multi-valued. The color of a bird ...
