The first computer monitors used single colour cathode-ray tubes (CRT). The beam from the electron gun in a CRT can be directed in two dimensions at the phosphor screen and often varied in intensity to control the brightness of the spot. CRT-based monitors typically use a raster scan at a fixed rate to rapidly draw a bitmap image. If the refresh rate (maximum frame rate) is high enough then the image is perceived as flicker-free.
Modern computer monitors use backlit LCD or LED.
The CRT can also be used in other ways, early radar displayed targets (reflections) using a slow circular radial scan and longer persistence phosphors. The animation below shows an example from Radar and Its Applications (1962).
CRTs were also used for vector monitors where the beam was directed to create a line-based image rather than using a fixed, grid-like scan pattern. Since these lines were not limited by the pixels of a (low resolution) bitmap display they could produce higher definition graphical output. This style of display was adopted for a few arcade games, Asteroids is a well-known one. A much earlier game called Spacewar! preceded Asteroids. Colour was introduced by Atari for the Star Wars (1983) arcade game but as the cost of video memory decreased and the resolution of video cards improved the demand for and use of vector displays diminished.
The first oscilloscopes used CRTs too - these are now sometimes referred to as cathode-ray oscilloscopes (CRO) to differentiate them from the modern flat panel digital storage oscilloscopes (DSO). Oscilloscopes are normally used for inspecting electrical signals but they can also be used to display images in various ways.