The Cyber Physical System (CPS), Internet of Things (IoT) and Digital Twin are all central concepts in Industry 4.0, often used interchangeably in discussions about Industry 4.0 and smart manufacturing. Each refers to a representation of a piece of equipment in cyber space. Such representations are of central importance in Industry 4.0 and for smart manufacturing, since they provide access to real-time operational data of the represented equipment. Use of this data ranges from machine operational status and compiling important KPIs, like OEE, MTBF, MTBA, etc., to big data analytics and machine learning applications, such as predictive maintenance. It is therefore worthwhile to examine what each means and how they relate to each other.