Non-equilibrium thermodynamics is a branch of thermodynamics that deals with thermodynamic systems that are not in thermodynamic equilibrium. Most systems found in nature are not in thermodynamic equilibrium; for they are changing or can be triggered to change over time, and are continuously and discontinuously subject to flux of matter and energy to and from other systems and to chemical reactions. Non-equilibrium thermodynamics is concerned with transport processes and with the rates of chemical reactions. Many natural systems still today remain beyond the scope of currently known macroscopic thermodynamic methods.
The thermodynamic study of non-equilibrium systems requires more general concepts than are dealt with by equilibrium thermodynamics. One fundamental difference between equilibrium thermodynamics and non-equilibrium thermodynamics lies in the behaviour of inhomogeneous systems, which require for their study knowledge of rates of reaction which are not considered in equilibrium thermodynamics of homogeneous systems. This is discussed below. Another fundamental difference is the difficulty in defining entropy in macroscopic terms for systems not in thermodynamic equilibrium.
Read more about Non-equilibrium Thermodynamics: Overview, Basic Concepts, Stationary States, Fluctuations, and Stability, Local Thermodynamic Equilibrium, Entropy in Evolving Systems, Flows and Forces, The Onsager Relations, Speculated Thermodynamic Extremum Principles For Energy Dissipation and Entropy Production, Applications of Non-equilibrium Thermodynamics