The thermodynamic processes can be classified into reversible and irreversible processes. The processes that can be restored to their initial state are called reversible processes. It is only possible if the process is in quasi-static equilibrium, i.e., it takes place in infinitesimally small steps, and the system remains at equilibrium However, these are ideal processes and do not occur naturally. An ideal system undergoing a reversible process is always in thermodynamic equilibrium within itself and its environment. Any change in its state that occurs can then be reversed by changing the system's conditions by an infinitesimal amount. For instance, the heat flow between two bodies having infinitesimal temperature differences can be reversed by making only a very minute change in one's temperature.

On the contrary, an irreversible process is usually what we encounter in reality these processes, the system, and the surroundings cannot be restored to their original states. All natural processes are irreversible. The sign of an irreversible process comes from the finite gradient between the states occurring in the actual process. For example, when heat flows from one object to another, there is a finite temperature difference (gradient) between the two objects. More importantly, the system most likely is not at equilibrium or in a well-defined state at any given moment of the process. This phenomenon is called irreversibility.

Intuitively, heat always flows from a hotter object to a colder one. When we hold an ice cube in our hands, we feel cold because the heat has left our hands and transferred into the ice cube. The opposite is true when we hold one end of a metal rod while keeping the other end over a fire. Based on the experiments on spontaneous heat transfer, the following statement summarizes the governing principle: Heat never flows spontaneously from a colder object to a hotter object. This statement is referred to as the Clausius statement of the second law of thermodynamics.