The electric power sector is one of the later sectors in adopting digital twins and models in the loop for its operations. This article firstly reviews the history, the fundamental properties, and the variants of such digital twins and how they relate to the power system. Secondly, first applications of the digital twin concept in the power and energy business are explained. It is shown that the trans-disciplinarity, the different time scales, and the heterogeneity of the required models are the main challenges in this process and that co-simulation and co-modeling can help. This article will help power system professionals to enter the field of digital twins and to learn how they can be used in their business.