Cavity quantum electrodynamics is a subfield of quantum optics and atomic physics that investigates the fundamental interaction between light and matter when the matter system (such as an atom or artificial atom) is confined within a resonant optical cavity. This confinement modifies the electromagnetic vacuum and the density of photon modes, enabling phenomena like enhanced or suppressed spontaneous emission, strong coupling between the matter and cavity modes, and serving as a crucial platform for exploring quantum measurement, entanglement generation, and quantum information processing protocols.