CRT: Cathode Ray Tube. The cathode ray tube was used in televisions and computer screens. Inside the tube, an electron gun produces a beam of electrons which sweeps over the flat surface to produce an image.
These were the original screens that were used in the first television sets. The electrons are emitted within a vacuum inside the tube. They are accelerated by electrodes and focused into a beam that strikes a phosphorescent layer inside the screen at the other end of the tube. Although they are quite cheap and can generate richly coloured images viewable from all angles, CRT screens are cumbersome and fragile and are now obsolete.