Quantum mechanics or quantum theory is a field of physics, which appeared due to inability of the classical concepts "particle" or "wave" to fully describe the behavior of quantum-scale objects. It was first shown in double slit experiment that elementary particles and quantum entities exhibit the properties of wave when we don't observe them and behave like particles when they under observation. This fact is called a wave-particle duality.This phenomena is lying in the basic of quantum theory. In its formalism all the information about a particle is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. This function evolves according to the Schrödinger equation. For particles with mass this equation has solutions that follow the form of the wave equation. Propagation of such waves leads to wave-like phenomena such as interference and diffraction.
Predictions of quantum mechanics have been verified experimentally to an extremely high degree of accuracy. Quantum mechanics is essential to understanding the behavior of systems at atomic length scales and smaller. Objects on larger scales are obey classical mechanics, as quantum effects are negligible. It means that classical mechanics is just an approximation of quantum mechanics for large systems of objects.