A thermodynamic system is a macroscopic volume in space, the adventures of which are to be studied according to the principles of thermodynamics, along with its walls and surroundings. Not just any physical system is a thermodynamic system, but only those that can be adequately described by thermodynamic variables, such as temperature, entropy, internal energy, and pressure.
A particular choice of thermodynamic variables that just specifies the internal state of a system constitutes its set of state variables. The set of relevant values of those is called the thermodynamic state of the system. In addition to the state variables, for an adequate thermodynamic account, there is further needed a special kind of function of them, called a state function. For example, if the state variables are internal energy, volume, and mole amounts, the needed further state function is entropy. It has been found by experience that these quantities are inter-related by one or more functional relationships called equations of state. Thermodynamics is largely about the restrictions on the possible equations of state that have been found to be imposed by the laws of thermodynamics, through that further function of state.