The fundamental basis for Quantum Mechanics is the idea that all energy (and, therefore, all mass) is only available in discrete multiples of a single unit called a quantum. Quantum Mechanics is, therefore, the study of how physical systems like atoms interact given that rather bizarre and unexpected constraint.

One of the beautiful things about Quantum Mechanics is that idea of the valence of electrons which is the basis for all of Chemistry is derivable directly from quantum mechanical theory.

An important philosophical consequence of Quantum Mechanics is that the quantizing of energy necessitates that all particles are described by a waveform which is something like the complex square-root of a probability distribution. That is, the fundamental description of all matter is probabilistic and not deterministic. The Strong Law of Large Numbers, of course, applies to the probabilistic events which take place at the atomic level, and so our macroscopic experience of the universe is fairly deterministic. Nevertheless, the Newtonian ideal that universe is deterministic and predictable was destroyed by Quantum Mechanics.

Also, several creatures from Nethack whose attack can effect a character's location and the consumption of whose corpses can effect a character's speed.