Entity Type
An entity type describes a class of entities with the same set of properties1. For example, a person has a name and a birthday.
An entity can have one or more entity types.
Entity types can be organized in a hierarchy. An "Employee" is a "Person. The "is a"-relationship between entity types describes that an employee can have all the attributes of a person.
Entity Types can also build a graph. If you specify the types of values for relational properties. For example, a person lives at a location.
Formal entity types can use a controlled vocabulary 2 to describe entities. For example, schema.org describes a set of entity types for use in linked data. The formal description of entity types allows combining entities from different sources.
Entity types also allow the verification or corroboration of data. For example, if "John Doe" has schema.org descriptions on five websites, and three of the websites agree on a birthday, I can recognize this as a fact of the entity. As the birthday property helps to distinguish two people, I can assume that if the other two websites agree on another birthday, then they might refer to a different person with the same name.
Footnotes
A property is a specific attribute of an entity. A property describes an entity by its value.↩
A controlled vocabulary is a list of words to tag units of information in a text or document. Controlled vocabularies can be used for information retrieval.↩