Macroscopic descriptions
Thermodynamics describes systems using macroscopic quantities such as temperature, pressure, volume and internal energy. Calculations need not follow every molecule. They do require a defined boundary, specified conditions and an account of whether energy or matter can cross between the system and its surroundings.
References: [1]
The first law
The first law balances internal-energy change against heat transfer and work. Heating a gas may produce expansion as well as temperature change. Internal energy is a state function, while heat and work depend on the route taken between initial and final equilibrium states.
References: [1]
The second law and heat engines
Energy conservation alone does not determine which processes can occur spontaneously. The second law rules out a cyclic engine whose only effect is converting heat from one reservoir entirely into work. Refrigeration likewise requires an external contribution to transfer heat from colder to hotter surroundings.
References: [2]
Entropy and statistical descriptions
Entropy quantifies constraints associated with irreversibility. It can decrease locally through exchanges with surroundings, while the total for an isolated system does not decrease. Statistical descriptions connect these macroscopic rules with microscopic arrangements, explaining why conserved energy need not remain equally available for useful work.