This chapter has the objective of describing some fundamentals of thermodynamics and applications previously explained in literature and classical textbooks. These are used to propose analyses based on the second law of different energy conversion processes. It is essential to highlight that from the classical point of view, defining energy is quite complex since there are several types and qualities. For a closed system, one can increase the energy from frontier interactions such as heat and work (two kinds of energy transfer), which are not properties; they are a function of the process. At the same time, the internal energy associated with the average kinetic energy of the molecules can be increased or decreased (thermodynamic property). In our daily life, when we eat chocolate, we let some mass enter our body, carrying the chemical energy of the glucose molecules. Hence, the energy can appear in several modes and qualities, implying that 1 J (one Joule) of energy does not always conduct to the same effect. Here comes the second law, with entropy and its unity Joule per Kelvin (J/K), bringing all kinds of energy to the same thermodynamic basis. Moreover, the term exergy converts all types of energy to a capability to perform one useful effect by means of a linear combination of the first and second laws of thermodynamics. It is more intuitive than the concept of entropy, and wherever there is a deviation from the environment, there is a capability to perform work until the system reaches thermal, mechanical, and chemical equilibrium with the environment. At the same time, the entropy of the system is always increasing while it is approaching the equilibrium.