Thermodynamics is the study of the energy and work of a system. It deals with only the large scale response of systems, which are observable and measurable. Thermodynamics has three principal laws: the zeroth law, the first law and the second law. Each law defines thermodynamic properties that help in understanding and predicting the operations of physical systems.
The zeroth law involves the basic definitions of thermodynamic equilibrium, which lead to the broad scale definition of temperature. It states that if two systems are thermally in equilibrium with another system, they are also in a thermal equilibrium with each other. The first law states that a system's change in energy is equal to the difference of the amount of energy transferred into the system and the work done by the system.
The first law is taken as a definition of internal energy, and it introduces a state variable known as enthalpy. It allows for the possible existence of many states of a system, but experience shows that only certain states exist. The second law defines a state variable known as entropy. It stipulates that the cumulative entropy of a system and its environment cannot decrease. The entropy can remain constant in case of a reversible process, but it always increases for an irreversible process.