The four laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems. The laws describe how these quantities behave under various circumstances, and forbid certain phenomena (such as perpetual motion).
The four laws of thermodynamics are:
Zeroth law of thermodynamics: If two systems are in thermal equilibrium separately, with a third system, they must be in thermal equilibrium with each other. This law helps define the notion of temperature.
First law of thermodynamics: Because energy is conserved, the internal energy of a system changes as heat flows in or out of it. Equivalently, perpetual motion machines of the first kind are impossible.
Second law of thermodynamics: The entropy of any isolated system never decreases. Such systems spontaneously evolve towards thermodynamic equilibrium — the state of maximum entropy of the system. Equivalently, perpetual motion machines of the second kind are impossible.
Third law of thermodynamics: The entropy of a system approaches a constant value as the temperature approaches absolute zero. With the exception of glasses the entropy of a system at absolute zero is typically close to zero, and is equal to the log of the multiplicity of the quantum ground state.
The laws of thermodynamics are important fundamental laws in physics and they are applicable in other natural sciences.