Quantum mechanics is an area of physics dealing with phenomena where the action is of the order of the Planck constant. The Planck constant is a very tiny amount and so this domain of physics is typically on the distance and momentum scale of atoms and elementary particles in general. Action is a general physical concept related to dynamics and is most easily recognized in the form of angular momentum. The most tangible way of expressing the essence of quantum mechanics is that we live in a universe of quantized angular momentum and the Planck constant is the quantum. A tangible result of the quantization of angular momentum is the existence of discrete electron orbitals, each with a principal quantum number and each orbital with an associated angular momentum that is an integer multiple of the Planck constant. Quantum mechanics has many implications on the microscopic scale, some of which are obscure and even counter intuitive.
Classical physics explains matter and energy at the macroscopic level of the scale familiar to human experience, including the behavior of astronomical bodies. It remains the key to measurement for much of modern science and technology. On the other hand, at the end of the 19th century scientists discovered phenomena in both the large (macro) and the small (micro) worlds that classical physics could not explain. Coming to terms with these limitations led to the development of quantum mechanics, a major revolution in physics. This article describes how physicists discovered the limitations of classical physics and developed the main concepts of the quantum theory that replaced them in the early decades of the 20th century.[note 1] These concepts are described in roughly the order they were first discovered; for a more complete history of the subject, see History of quantum mechanics.[1]
Some aspects of quantum mechanics can seem counter-intuitive or even paradoxical, because they describe behavior quite different than that seen at larger length scales, where classical physics is an excellent approximation. In the words of Richard Feynman, quantum mechanics deals with "nature as She is – absurd."[2]
Many types of energy, such as photons (discrete units of light), behave in some respects like particles and in other respects like waves. Radiators of photons (such as neon lights) have emission spectra that are discontinuous, in that only certain frequencies of light are present. Quantum mechanics predicts the energies, the colours, and the spectral intensities of all forms of electromagnetic radiation.
Quantum mechanics ordains that the more closely one pins down one measurement (such as the position of a particle), the less precise another measurement pertaining to the same particle (such as its momentum) must become. This is called the uncertainty principle, also known as the Heisenberg principle after the person who first proposed it.
Put another way, measuring position first and then measuring momentum does not have the same outcome as measuring momentum first and then measuring position; the act of measuring the first property necessarily introduces additional energy into the micro-system being studied, thereby perturbing that system.
Even more disconcerting, pairs of particles can be created as "entangled twins." As is described in more detail in the article on Quantum entanglement, entangled particles seem to exhibit what Einstein called "spooky action at a distance," matches between states that classical physics would insist must be random even when distance and the speed of light ensure that no physical causation could account for these correlations.[3]

Bibliography:

1. Bernstein, Jeremy (2005). "Max Born and the quantum theory". American Journal of Physics 73 (11): 999. doi:10.1119/1.2060717.
2. Beller, Mara (2001). Quantum Dialogue: The Making of a Revolution. University of Chicago Press.
3. Bohr, Niels (1958). Atomic Physics and Human Knowledge. John Wiley & Sons. ASIN B00005VGVF. ISBN 0-486-47928-5. OCLC 530611.