An Entity Type is a category. An Entity is an individual instance of a given Entity Type. The same is said for Document Types vs. Documents: One is the category, the other is an instance of that category.

An Entity is an independent and unique item that can be defined. When we speak of an Entity, we normally speak of some aspect of the real world which can be distinguished from other aspects of the real world. An Entity may be a physical object such as a house or a car, an event such as a house sale or a car service, or a concept such as a customer transaction or order. 

A Relationship captures how Entities and/or Documents are related to one another. Relationships can be thought of as verbs, linking two or more nouns.

Examples: an  owns  relationship is between a company and a computer; a  supervises relationship is between an employee and a department; a  performs  relationship is between an artist and a song; a  proved relationship isbetween a mathematician and a theorem.”