True although as I mentioned, it is a very messy area. Here is a one-minute tutorial.
LCD technology unlike Plasma does not product light. The LCD itself is a programmable light filter. It can let in a bit of light or a lot. The source of the light is a diffused panel behind the LCD which traditionally used fluorescent bulbs (CCFL).
With the advent of "white" LEDs, a trend started a few years ago to use them as a replacement for CCFL. LEDs have the advantage that they can be dimmed and with it, bring power saving.
In the simplest form, LEDs can take the place of CCFL but with dimming capability (less light at night and more during the day). Next step up is to change that light level with picture content leading to the super misleading contrast ratios of millions to 1 where they measure the picture with LED at max, then put up a black screen and measure it with the LEDs all the way down. Of course, nothing is realistic about that (and hence the reason you should ignore dynamic contrast ratio numbers).
Because LEDs are smaller, they can also be put on the edge instead of behind the panel. Diffusers are then used to light up the whole panel. This results in very thin displays but unfortunately, you get uniformity issues where parts of the screen are brighter than others. Effects like "flashlighting" are common now.
A second variation is to put the LEDs in the back, but dynamically adjust clusters of them. That way, you can light up the dark parts of the screen differently than the light parts. To work though, you need many regions and due to cost issues, far fewer than optimal is used. You get better performance in these so called "local dimming" sets but all is not perfect. These add bulk to the unit which is a negative in marking the unit (people think thinner is better). They also make the unit more expensive. And unfortunately still add artifacts of their own where you get screen blotchiness. Given the economic situation, there are fewer offerings this year of this category product than last.
