The system is any region of space, which is under investigation.
The surroundings are considered to be all other things, which can interact with the system.
Anything that separates the system and surroundings is called boundary. The system may be classified into three categories depending upon its characteristics. These are (1) closed system (matter can neither be added nor removed from it), (2) open system (matter can be added or removed), and (3) isolated system (neither energy nor matter can be added or removed).
A system under study may include one substance or more than one substance in any of the three states of matter. The state of the system can be prescribed by stating the values of state variables such as amount of substance, pressure, volume, temperature, composition, etc.