In Data Modeling, an entity-relationship model (ERM) is a representation of structured data; entity-relationship modeling is the process of generating these models.
The end-product of the modeling process is an entity-relationship diagram (ERD), a type of conceptual data model or semantic data model.
The first stage of information system design uses these models to describe information needs or the type of information that is to be stored in a database during the requirements analysis.
The Data Modeling Technique can be used to describe any ontology (i.e. an overview and classifications of used terms and their relationships) for a certain universe of discourse (i.e. area of interest).
Entity & Entity sets
An Entity is an object that exists and is distinguishable from other objects. For instance, John Harris with S.I.N. 890-12-3456 is an entity, as he can be uniquely identified as one particular person in the universe.
An entity may be concrete (a person or a book, for example) or abstract (like a holiday or a concept).
An entity set is a set of entities of the same type (e.g., all persons having an account at a bank).
Entity sets need not be disjoint. For example, the entity set employee (all employees of a bank) and the entity set customer (all customers of the bank) may have members in common.
Entity-relationship diagrams don't show single entities or single instances of relations.
Rather, they show entity sets and relationship sets.
Example: a particular song is an entity. The collection of all songs in a database is an entity set.
The eaten relationship between a child and her lunch is a single relationship.
The set of all such child-lunch relationships in a database is a relationship set.