A set of entities that have the same attributes is called an entity type. Each entity type in the database is described by a name and a list of attributes. For example an entity employee is an entity type that has Name, Age and Salary attributes.
The individual entities of a particular entity type are grouped into a collection or entity set, which is also called the extension of the entity type.
An entity is a thing in the real world. It may be an object with a physical existence or an object with a conceptual existence. A set of these entities having same attributes is entity type and collection of individual entity type is an entity set.