LCDs

Twisted nematic liquid crystals are typically used to make Liquid Crystal Displays (LCDs). These liquid crystals have an interesting feature: when an electric current is applied to them their twisted shape will begin to untwist. This untwisting allows the controlled passage of light on how much the liquid crystal is untwisted.

To make a basic LCD display you take two pieces of polarized glass that are at right angles to each other and sandwich them in between a layer of liquid crystals and electrode. As light passes through the polarized glass, the twisted nematic liquid crystals will guide the light through at an angle so it passes through the second sheet of polarized glass. and light will reach the display screen. If the electrode is applying a voltage to these liquid crystals, then they will untwist and the light will pass the liquid crystals at an angle so they are not able to go through the second sheet of polarized glass, thus light will not reach the display screen. Varying degrees of voltage can control how much light passes through the display screen.

Color LCD TVs usually have an active matrix system, which uses a series of thin film transistors and capacitors to apply a voltage to the pixels in a TV. These TVs have red blue and green filters that can act as sub filters for every pixel. The transistors are sensitive enough to create 256 levels of brightness, meaning you can get 256 shades of red, blue, and green in every pixel, and that means you can have (256 x 256 x 256) 1.6million colors in every pixel. Needless to say this gives you a sharp images.

http://electronics.howstuffworks.com/lcd.htm