Short for Cathode-Ray Tube, CRT is the electron beams within a monitor that move across your screen either interlaced or non-interlaced hiting phosphor dots on the inside glass tube. To the right is an example of the inside of a computer monitor including the CRT, thanks goes out to Jeff7 for sharing this image.
Within the CRT are three electron guns, red, green and blue. Each of these guns streams a steady flow of electrons, left to right, for each line of your monitor. As the electrons hit the phosphors on the CRT, the phosphor will glow certain intensities. As a new line begins, the guns will then begin at the left and continue right; these guns will repeat this process sometimes thousands of times until the screen has been completely drawn line by line. Once the phosphors on the CRT have been hit with an electron they will only glow for a short period of time; because of this, the CRT must be refreshed, which means the process will be repeated as explained above. If the video card's refresh rate is not set high enough, you may encounter a flicker or a noticeable steady line scrolling from the top to the bottom of your screen. If you are encountering this issue, resolutions have been described on our video card page.