ER approach was introduced by Prof. Peter Chen in TODS 1, 1976. In this video, we will see ER diagram in detail. Basically Entity Relational diagram is used to model the logical view of the system from data perspective which consists of these components:
1) Entity
2) Entity Type
3) Entity Set

An Entity can be considered as an object with a physical existence – a particular person, car, house, or employee – or it may be an object with a conceptual existence – a job, a company, or a university.

An Entity is an object of Entity Type and set of all entities is called as entity set. e.g.; E1 is an entity having Entity Type Student and set of all students is called Entity Set.

Relationship and Relationship type (or Relationship set) : A relationship type represents the association between entity types. In ER diagram, relationship type is represented by a diamond and connecting the entities with lines.

Attribute: Attributes are the properties that define the entity type. For example, _No, Name, DOJ, Age, Address, Mobile_No are the attributes that define entity type Student. In ER diagram, an attribute is represented by an oval shape.

An ER diagram is used to analyze to structure of the Database. It shows relationships between entities and their attributes. An ER model provides a means of communication. Express the number of entities to which another entity can be associated via a relationship set. Most useful in describing binary relationship sets.

For a binary relationship set the mapping cardinality must be one of the following types: 
1) One to one 
2) One to many 
3) Many to one 
4) Many to many

Read related article here:
https://www.geeksforgeeks.org/introduction-of-er-model/
https://www.geeksforgeeks.org/er-diagram-of-a-company/