The cathode ray tube (CRT) is a vacuum tube containing one or more electron guns (a source of electrons) and a fluorescent screen.
A CRT has an internal or external means to accelerate and deflect the electron beam which is used to form images in the form of light emitted from the fluorescent screen. The image may represent electrical waveforms (oscilloscope), pictures and text (television, computer monitor), radar targets and others. For computer use the standard interface was VGA.
The single electron beam can be processed in such a way as to display moving pictures (Video). Multiple beams can produce pictures in natural colors. Three beams are often used to represent the three primary colors of light (RGB).
The CRT uses an evacuated glass envelope which is large, deep, heavy, and relatively fragile. A thick front glass, typically epoxy-bonded, face-plates is used to help protect the display from breaking when hit. Display technologies without these disadvantages, such as flat plasma screens, liquid crystal displays LCD, DLP, Oled displays have replaced CRTs in many applications and are becoming increasingly more common.
 How it works
The electron beam is emitted from the neck of the tube and is deflected on its way to the face of the screen. The deflection can be done electronically or magnetically. The beam lights a phosphor spot on the screen at the point it hits and phosphor persistence allows the spot to remain lit for a short time even after the electron beam moves away from the spot. By controlling the location that beam hits the screen an image can be produced. It must be refreshed often to remain visible for the user.
There are two methods of controlling the display. The deflection can be controlled such that an image is drawn on the screen much like a pencil would be used to draw a picture. This method is used for Oscilloscopes. The second method is to send the beam in a fixed pattern across and down the screen to create a raster image that covers the whole screen display area. (Please see graphics for more on how a raster image displays a picture.) The darkness of the spot is what makes the image appear for the user. It is controlled by a signal used to determine the brightness of the spot itself. Most computers and TV's use this method. It is the only method used for motion video. Again the image must be redrawn rapidly to appear long enough for the user to see it.
Color is obtained by having different phosphor areas on the screen. The phosphor will light up in the predetermined color when hit with the beam. There are thousands of these phosphor dots (or lines in some implementations) on the screen surface and are used to create the picture. There are 3 beams in the color system with each independently controlled in brightness by the input signals. The 3 beams of color are Red, Green, and Blue. They are close enough together for the eye to merge the results to look like one image with a mix of colors such that many different colors can be represented using only the 3 primary colors.
Note that the equivalent pixel for the signal on a CRT is not square. The number of lines vertically is fixed and easy to compute the horizontal component is generated by a rapidly moving dot that must the modulated with the image. The fact that the beam is moving results in a rectangular pixel where the horizontal size 1.2 times the vertical size (actually a ratio of 40:33). This is factored into the signal processing so that the image will have the correct aspect ratio.