Quantum mechanics QM is a branch of physics which deals with physical phenomena at microscopic scales, where the action is on the order of the Planck Constant. Quantum mechanics deviates from classical mechanics mainly at the quantum realm of atomic and subatomic length scales. Quantum mechanics offers a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. Quantum mechanics is the non-relativistic limit of Quantum Field Theory (QFT), a theory that was built up later that united Quantum Mechanics with Relativity Theory.
Quantum mechanics is the body of scientific values that elucidates the behavior of matter and its interactions with energy on the scale of atoms and subatomic particles and how these happenings could be linked to daily life.