Quantum computers use various forms of quantum-level processs as an explicit part of their hardware. They make extensive use of qubits physical elements that are in multiple superimposed states, sort of 0 and 1 at the same time. Quantum effects mean that effectively the same pieces of physical hardware simultaneously perform multiple versions of the same calculations, in contrast to classical parallel processing where several physically dstinct processors do work on parts or variants of an algorithm at the same time.
Quantum computers may be used to model quantum-level effects in physics, but can also be used in other kinds of algorithms. In particular many of the techniques used in cryptography will become insecure as a quantum computer can effectively try billions of options of a key similtaneously. In AI, one can imagine that certain kinds of neural network could benefit and one of the technologies for reservoir computing is quantum; however practical applications are probably still some years off
Note that conventional computers may experience quantum effects such as tunneling between adjacent tracks, but this is usually regarded as a problem (indeed is one of the limits of conventional electronics), whereas quantum computers make deliberate use of these kinds of effect.
Used in Chap. 24: page 376