An HDTV is made up of an array of pixels. There are 1920 horizontal pixels and 1080 vertical pixels. That means there are 2073600 total pixels on a screen.
On an LCD (all current LED TV's are LCD's). There is a panel that lets light through each pixel location. This means the light is actually generated by a back light. In a traditional LCD, there are cold cathode tubes (similar to a fluorescent lights) behind the screen on the edges. The down side to this configuration is that the LCD panel can not block 100% of the light, some always escapes, so they can not show true black. The other problem is that since the light source is on the edges, the blacks will be less black along the edges (if you show a black screen, you will see a gradient of grey that gets lighter towards the edges).
An LED lit LCD replaces the cold cathode light with LED's. LED's have an advantage that they are brighter, smaller, more reliable, and more power efficient. However, most LED lit TV's don't offer any increase in picture quality because the lights are still along the edges (this is how they make them so thin).
There is another kind of LED lit LCD that is more expensive and a little rarer. It's a full array back lit screen. What this means is that there is an array of LED's directly behind the screen. Only the areas of the screen that are showing something are lit up. This makes the blacks much better. It's still not perfect. If you have a bright image in the middle of a dark screen, you will see a halo of grey in the black around the image. The more LED's you have the better the picture. But you rarely see more than 500 or so back there.
Another problem with LCD's is that the whole panel must be refreshed each time the picture changes. Progressive scan pictures have 60 frames a second which means that the LCD panel needs to refresh at least that many times. All of them will do this and some will do 120 or 240 times a second. However, what actually maters is the pixel response time (and it's almost never given). You can only tell if this is a problem by watching fast movement on the TV.
On a Plasma TV, each pixel is individually throwing off light. This means that individual pixels can be updated without refreshing the whole screen and true blacks can be shown. However, because of this they use more power; are heavier; are thicker; and aren't as bright. Also, since they require glass (LCD's don't but often have it anyway) they have more problems with glare.
The final thing to consider is 3D. Most 2012 models are going to have 3D whether you want it or not (it doesn't cost anything to add it in to the TV). There are two modes of 3D in the home. There is RealD passive and Active Shutter. Active shutter glasses work by turning on an LCD panel in one eye of the glasses. This blocks the light to one eye and allows each eye to see a different image. The downside is that the glasses require power, are expensive, and the picture is subject to cross talk. Some LCD's and all Plasmas use this technology. Passive 3D works by showing alternating lines of polarized picture and black. The glasses only allow in one kind of polarized light into each eyes. So on every frame you get 540 lines going to each eye. This means that the picture isn't quite as sharp. But there is no cross talk and the glasses are dirt cheap (you can use the ones they give you for free at the theater). On a screen 42" or smaller, you probably can't even see the difference.
Hope I answered all your questions.
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