Thermodynamics is a branch of science that deals with the relationship between heat and other forms of energy. A part of the universe where observations are made is called system. The surrounding of a system is the part of the universe that does not contain the system. Based on the exchange of energy and matter, thermodynamic systems can be classified into three types: closed system, open system, and an isolated system. To know more about the topics difference between the system and surrounding, the difference among closed, open and isolated systems, Internal energy, heat, work, First law of thermodynamics, Calculation of energy change, Correlation, Hess’s law, Extensive and intensive properties, are explained in this chapter.