CRT is the short form of Cathode-Ray Tube. A Cathode-Ray Tube as the name suggests is a vacuum tube that houses an electronic gun. Sometimes there is more than one electronic gun in a CRT. On top of the electronic gun, there is a phosphorescent screen.
The major use of CRT is for the projection or the display of images. This mainly happens in the televisions and the computer monitors. The images you see on computer monitors or the televisions are as a result of the use of a CRT.
CRT displays images by modulating, deflecting and accelerating an electric beam onto the screen. These images are displayed as a representation of electrical waveforms, oscilloscopes, pictures or radar targets.
CRT has also been used in the past as memory devices. To create an image the CRT controls the intensity of the three primary color beams. These are red, blue and green. This is how CRT varies the images.