An entity relationship diagram (ERD) is a representation of data within a domain. It consists of entities as well as relationships between entities.
An entity can be a tangible, physical object such as a school or student, or a concept such as a reply or a transaction. Entity can be identified by extracting objects that are relevant and meaningful to the problem domain and the system to develop. In entity relationship modeling, the term entity has synonyms "table", "database table", "entity-type". Yet, entity is the most commonly used term. Each entity brings along a set of columns, which are the properties of the entity the attributes belong to. For instance, entity Student has name, address and grade as columns (synonyms: attributes, properties, fields). Every entity must have at least one attribute that can be used to uniquely identify the entity, which is known as the entity's primary key(s).
Relationships are capable in linking up entities. Typical examples: one-to-one, one-to-many, many-to-many. The proper use of relationship is important in showing HOW entities are related. For instance, one-to-many relationship must be used for modeling the fact that 'one school has many students'.
