Cathode rays are a stream of high-speed electrons emitted from the heated cathode of a vacuum tube. In a video monitor, the electron beam is focused and diverted by electric fields to scan the surface at the opposite end of the vacuum tube which is lined with phosphorescent material. When the high-speed electrons hit the lining, light is emitted. The video signal is used to control the intensity and scanning position of the electron beam to form a video image on the TV screen.
CRTs have a pronounced triode characteristic which leads them to have a pronounced gamma response. Fortuitously, this functioned as a form of contrast compression in early television systems, and this gamma response exists today in all digital video systems.