The objective of this paper is to propose a categorization of the objects defined under the term “Digital Twins” in the literature, to reduce the misuse of this term and to facilitate the understanding of the developments in this scientific field. A review of the literature on the categorization and classification of Digital Twins has been carried out. A classification into four distinct categories was made, using a decision tree. These four categories of objects grouped under the term Digital Twin were then explained, presenting their functions as well as their frameworks of use. The classification criteria used for the realization of this study focus only on the existing links between the physical and virtual parts of the Digital Twins, from which will derive their functions. This document represents one of the first attempts to classify Digital Twins, and the first to propose a tool to facilitate the reading of articles on this subject, in the form of a decision tree