What is quantum mechanics?

In a more general sense, the word ​“quantum” can refer to the smallest possible amount of something. Quantum mechanics  is a fundamental theory in physics that describes the behavior of particles such as atoms, electrons, photons and almost everything in the molecular and sub-molecular realm.  Quantum mechanics is the best tool we have to understand how the universe works on its smallest scales. Heisenberg's Uncertainty Principle is one of the most celebrated results of quantum mechanics and states that one (often, but not always) cannot know all things about a particle (as it is defined by it's wave function) at the same time.