What do you mean by entity types and entity sets?
An entity type defines a collection (or set) of entities that have the same attributes. Each entity type in the database is described by its name and attributes. Fig. 1.9 shows two entity types, named EMPLOYEE and COMPANY, and a list of attributes for each. The collection of all entities of a particular entity type in the database at any point in time is called an entity set. The entity set is referred to using the same name as the entity type. For example, EMPLOYEE refers to both a type of entity as well as the current set of all employee entities in the database.