Liquid crystal display (LCD) monitors were early flat panel displays but they are more common, today in calculators and similar devices as 7 segment displays; although these displays are much simpler but they work off of the same principals.
In these displays microscopic crystals orientate themselves to a specific direction when a voltage is applied, this causes the light to be distorted and let more or less light through making it more or less grey. As the crystals themselves do not emit light a LED panel is placed behind the crystals. Also a polarising filter is placed behind and in-front of the crystals, these polarisers are often placed perpendicular to each other, in most cases this would cause all the light to be blocked but because the crystals changes the lights properties between the polarisers the light can make it through the second polariser.
Colour LCDs are made by layering multiple normal LCDs but with 3 colour polarisers R, G and B