A quantum computer differs from a classical computer in that it processes information using quantum states, primarily by utilizing principles such as entanglement and superposition in order to perform complex computations. Quantum computers currently exist today (IBM, Google, and other companies are all working towards building practical quantum computers), but still need to be worked on in order to be made more useful for commercial use.
Quantum computers, like classical computers, have two major components – hardware and software. Quantum software uses intricate algorithms in order to perform complicated simulations, multifaceted modelling, and other various computationally expensive tasks. On the other hand, quantum hardware employs principles across a multitude of STEM fields in order to maintain a physical environment in which quantum bits (qubits) can be used. Quantum hardware also allows for quantum software to be run.
Currently, the applicability of quantum computers is hindered by several factors. For example, decoherence occurs when the environment causes qubits to lose their states. Furthermore, it is very challenging to maintain the necessary conditions (such as extremely low temperatures) needed to perform quantum calculations, as even small temperature changes can fracture the already fragile stability of a quantum system. Hopefully, in the future, these difficulties will be overcome and quantum computers will allow humanity to make huge progress in not just physics, but a wide array of fields.