An example of a thermodynamic system is a brick. A brick is made up of many atoms which all have their own properties. All thermodynamic systems have two kinds of properties, "extensive" and "intensive". For the brick, the extensive properties are the ones you get by adding up all the atoms. Things like the volume, energy, mass, and charge are extensive because two of the same brick put together have twice as much mass as one brick. The intensive properties of the brick are the ones you get by looking at the average over all atoms. Things like temperature, pressure, and density are intensive because two of the same brick still has the same temperature as one brick alone.