You have asked a very general question -- I am not sure which answer you are looking for. I'll answer generally, and if you need more specific information, maybe you can ask a more detailed question, after seeing this.

Essentially, a computer screen has the task of making the internal state of a computer known to a human user. For this purpose, there are two techniques:

1. a map of colored bits is made in memory (pixels) which are displayed on a CRT or flat panel display -- each pixel (or pel) of the map is displayed by three colored dots on the display.

2. for moving 3-D images such as games, a display list is kept in memory which keeps a map of triangular facets, each of which is colored, has qualities of texture and reflection and can be hidden behind other triangles. This list is dynamically converted into an image map as the map is displayed on the screen.

In both cases, the display uses a trio of colored dots (can be short parallel segments or dots in a triangle), which can be displayed with predefined brightness. The three colors chosen approximate the dye used by your eye to discern color (the cone cells in the retina).

Since in normal humans there are three such dyes, only three colors need to be added to make a reasonable image -- with your brain filling in the inaccuracies. (Color blind people are missing one or more of these dyes in their eyes -- so they cannot distinguish some combinations of colors -- e.g. red/green colorblindness.) By the way -- although it is true that cats are colorblind (as we understand color) they have much better night vision. Ducks, by the way have 8 dyes in their eyes -- so have much sharper color discernment than humans. Their rainbow would likely have tens of distinct colors -- not just the 7 or 8 colors we see.

Physically, a CRT has three electron beams which are scanned across every set of dots on the front of the tube -- the beams can be modulated (the intensity of the beam can be changed quickly) so that each dot gets the right amount of brightness for that part of the image.

In a TFT panel display, a florescent panel light is mounted in front of an array of amorphous transistors which control liquid crystals. These crystals align themselves with an applied electric field (from the transistor) which changes the polarization of light through the pixel. When the polarization matches a special coating on the display, light is let through -- so the LCD acts like a variable shutter to the light coming from the panel behind it. Again trios of shutters are used to make any color out of just three. The colors are red, green and blue -- although yellow is a primary, you don't have a dye which responds to yellow -- instead, it triggers both the green and red dyes in your eye -- and you see yellow. The process is additive, unlike mixing paint, so that red and green make yellow, blue and green make cyan and red and blue make magenta. All three make white.

Basically, though, inside a normal cathode ray tube (CRT) monitor, which looks a lot like a television (because it is!), an electron beam is created inside the tube and directed by magnetic fields to hit a screen that glows when hit by the electron beam. The beam hits one small point on the screen at a time but scans through all the points on the screen more quickly than our brain notices. Sometimes you can see some odd effects on a T.V. due to the refresh rate. Try watching the tires rotating on a moving car on T.V. some time; you might notice something interesting.

Some of the newer monitors, such as on laptops, use a different technology like liquid crystal displays (LCD). In this case light is generated behind a panel that controls whether light gets through or not at each pixel. Color displays can also control whether red, green, or blue light, or combinations of those can get through. A really exciting technology is light emitting diode (LED) displays. In this case, very small LEDs would be on a screen and would probably use very little power. I don't know if this technology is quite there yet because it is difficult to make panels of really small LEDs and some colors are difficult. I guess the large screens at stadiums use LEDs. In that case, the screens are so big that you don't notice the individual LEDs, which are probably at least a few millimeters in diameter.

I like your question. It's one that people just don't ask enough because computer displays are so common to us. It's almost like asking 'why is the sky blue?'

Computer screen technology has come a long way since the blinking green monitors of the 1970s. Those were painful times, let me tell you. In terms of display types, there are actually three primary technologies in use today. These include the "cathode ray tube (CRT)", the "liquid crystal display (LCD)", and "thin film transistor (TFT)" displays. The latter technologies (LCD and TFT) are used in flat screen displays like laptops and panel monitors. You'll notice that these types of displays are quite expensive and therefore less common. They are growing in popularity, though, because they take up so little space! CRT technology is still the most common. It is used for most televisions and the majority of desktop computer monitors. I'll describe how this display works in the most detail.

Let's start with some basic notions about how the brain perceives images. In terms of display technology, the following principle is the most important and quite amazing. If you divide a picture into a big collection of small colored dots, your brain will reassemble the dots into a meaningful image. You've probably heard the word 'pixel' before : "My computer screen has a resolution of 800x600 pixels." What this means is that if you looked really close at your monitor, you would be able to see 480,000 dots, or pixels (800 multiplied by 600 = 480,000)! So, if you were able to light up each one of those dots with the proper color, your brain would actually see the huge collection of dots as an image. I think this is amazing. All three of the technologies (CRT, LCD, and TFT) rely on this principle, but each 'lights up' the pixels in different ways.

In the cathode ray tube (CRT), the "cathode" is a filament that spits out electrons when it is heated. Magnets are used to focus the beam of electrons and they are aimed at the backside of the screen you are watching. The special thing about the screen is that it is coated with a special material called a 'phosphor.' Phosphors are special because when they are hit with electrons they glow! So when we see an image of an apple on the computer screen, we are actually seeing the light given off by thousands and thousands of phosphors! The tricky part is controlling the electrons and making sure that they hit the right pixels so that the image looks like an apple and not some red blob.