What is quantum mechanics?

Quantum mechanics was constructed in the period of 1900 ∼ 1928 by a school of well-educated geniuses, some are only youngsters in their twenties. The quantum theory, though still being far from complete, has since essentially changed physics, consequently our world. In this article, after verbally explaining the meaning of quantum and mechanics, the theory of quantum mechanics will be presented with regard to the details of its construction, and concluded with some exemplary applications. To the current author, quantum mechanics is in no sense a revolution, rather it is simply a natural extension of classical mechanics. Quantum mechanics, and also the contemporary theory of relativity, should be the knowledge default for students of today. (author)