a flat screen TV has many meanings. before the invention of LCD and Plasma it was used to describe a TV with a flat tube.
however, nowadays it’s used to describe usually plasma units, but the same can be used for LCD TV’s.
plasma TV’s and LCD TV’s work in similar fashions but use vastly different technologies. Plasma TV’s use two panels of glass phosphers made of gas (usually xenon and neon). When electiricty is applied to electrodes in the glass…the gas glows. The brightness is controlled by varying the electric current. These screens respond VERY quickly to action and give a nice clear picture.
LCD TV’s use basically the same method..only rather than depending on glowing gas discharge..it has a layer of liquid crystals sandwiched between two plates of glass. as electricity is applied, the crystals align. polarized sheets on the front and back combined with the polarization effect of the crystals lining up, causes light to pass through. varying the voltage changes the brightness. however, once the crystals are charged, they have to remain charged for one full cycle, which depends on the model of the screen. most have gone down to mere milliseconds…but LCD’s can cause "ghosting" when you’re watching things with fast motion and also cannot produce a true black.
projection TV’s have two ways of working. Traditional projection units have three projectors positioned right below the screen positioned toward the back. the light is reflected toward the screen which is in fact a special kind of fresnel lens. this type of TV is slowly being replaced, however, they do still make them. the biggest problem with these units is convergence. since you’ve got three seperate projectors (red, green, blue), keeping them in perfect alignment as the unit gets older can be difficult and results in something similar to color bleeding.
the other type of projection TV is a DLP. DLP contains a microchip with a matrix of mirrors, one microscopic mirror for each pixel. light is then reflected off the mirrors as they’re rapidly turned on and off. most units are single chip and use a high-speed rotating color wheel that thanks to the flicker rate of our eyes, a color image is created. newer units have replaced the color wheel with colored LED’s which create a better image without the "rainbow effect". high quality units use three DLP chips and a prism to combine the images.
http://en.wikipedia.org/wiki/Comparison_of_display_technology has a comlete list of every type of display technology, thier uses, and articles how each one works.