Quantum mechanics [1] deals mainly with the extremely small such as atoms and subatomic particles. While it works on the macroscopic level as well, with few a exceptions; such as superconductorsand superfluids; the results are usually the same as Classical mechanics. It is mainly at the subatomic level that the differences become significant.
Thermodynamics is the branch of physics that deals with the movement of energy with in matter. It mainly deals with the movement of heat energy (hence the name), but that study has led the field to include more that heat. This is result of the fact that at the molecular level heat is just the energy of molecular motion. As a result Thermodynamics includes the affects of molecular motion.
In simple terms, the Second Law of Thermodynamics states that the entropy of a closed system will always increase, and a local decrease in entropy in an open system always results in a larger increase in external entropy.
The 2nd law, and the trend to disorder, is universal (Talk.Origins)