As one of the key technologies to promote intellectualization and digitalization, Digital Twin is a technical means based on simulation, with the integration of data and model as the core. It is real-time, reusable and intelligent. For digital twin technology, this paper briefly describes its development history and definition, proposes that digital twin consists of three basic components: physical layer, transmission layer and virtual layer, and explains the key technologies involved, such as sensing and monitoring, complex system modeling, big data analysis, etc. This paper summarizes the current research on digital twins, including theory, data and technology, describes the typical applications of digital twins in industrial manufacturing, smart cities, aerospace and other aspects, and finally looks forward to their development trends.