LCD

LCD HDTV can be defined or explained in a complex way.  I will try to explain LCD in a simplified manner.

LCD stands for liquid crystal display.  LCD displays use sheets of polarizing material. One layer is coated with a special polymer that holds the individual liquid crystals.  An electric current is passed through the individual crystals, which each crystal then acts like a shutter either allowing light to pass through or blocking the light to create images.  Individual LCD shutters open and close which allow a metered amount of the white light through. Each shutter is paired with a colored filter to remove all but the red, green, or blue  portion of the light.
LCD does not produce it's own light.  An external light source is needed to become visible to the viewer.  The light is usually a series of cold cathode fluorescent lamps although some use white or colored LED's.  LCD's are thin and don't require a lot of power to operate.  LCD TV pixels are turned off or on at a specific refresh rate.  This is where you get the 60hz, 120hz, or 240hz ratings from.  It is the rate in which the image is refreshed.
The lifespan of an LCD TV is debatable.  For the most post LCD TV's have a lifespan of 60,000 hours in a controlled environment.  Many manufacturers have claimed a lifespan up to 80,000 hours or more.  This is, of course, in a controlled environment and used continuously.  It is to note that LCD will have signs of "wear".  This is due to the light source which will dim over time causing the picture you see to dim also.  Therefore, the most important factor to the lifespan of an LCD is the lifespan of the light source.
Some manufacturers have started to product LCD TV's whose light source (lamps) can be changed thus giving you a "like new" picture after it has been changed.