Quantum mechanics is concerned with phenomena that are so small-scale that they cannot be described in classical terms, and it is formulated entirely in terms of statistical probabilities.  Considering one of the great ideas of the 20th century, quantum mechanics was created (mainly) by Niels Bohr, Erwin Schrodinger, Wener Heisenberg, and Max Born.  This led to a drastic reappraisal of the concept of objective reality.  Quantum mechanics explained the structure of atoms, atomic nuclei, and molecules; the behavior of subatomic particles; the nature of chemical bonds; the properties of crystalline solids; nuclear energy; and the forces that stabilize collapsed stars!  Quantum mechanics has also led directly to the development of the laser, the electron microscope, and the transistor.