In this chapter it is clarified what a thermodynamic system is and how its state can be described. Any system is separated from an environment by a system boundary as it is done in other technical disciplines, e.g. technical mechanics, as well. First, the permeability of the system boundary is categorised. However, after classifying the system and the system boundary, it is the next step to identify the internal state. This leads to so-called state values, e.g. pressure and temperature, that fix the state of a system. Our everyday experience shows, that the state can be varied by external impacts across the boundary.