Define the terms entity type and entity set.

A set or a collection of entities that share the same attributes but different values is known as an entity type. For example, the set of all publishers who publish books can be defined as the entity type PUBLISHER. A specific occurrence of an entity type is called its instance. For example, the publisher Hills Publications is the instance of the entity type PUBLISHER. An entity set is a collection of all instances of a particular entity type in the database at any point of time. Each entity is referred to by its name and attribute values. An entity type describes the schema (intension) for the entity set that shares the same structure. The entity set, on the other hand, is called the extension of the entity type.