The three laws of thermodynamics are: the zeroth law of thermodynamics, the first law of thermodynamics and the second law of thermodynamics. Each law explains physical properties of thermodynamic systems that help in understanding and predicting the operations of the system.
The zeroth law of thermodynamics incorporates fundamental definitions of thermodynamic equilibrium, which result in the broad scale definition of temperature. According to the law, two systems that are in a thermal equilibrium with another system are also in a thermal equilibrium with each other. The first law of thermodynamics associates the various forms of potential and kinetic energies of a system with the work the system can perform and heat transfer, and it defines the internal energy of a system. According to the first law, the change in the energy of a system is equal to the difference between the heat energy transferred into the system and the work done by the system. The first law also introduces enthalpy, which is a state variable. Many states of systems can theoretically exist according to the first law, but observation points to the existence of only a few. The second law of thermodynamics introduces entropy, another state variable. The law states that the cumulative entropy of a closed system always increases. The change in entropy can't be negative.