LED backlit TVs — or lets call them LED TVs for short — have been growing in popularity, and not just due to their cost having decreased over time. LED backlighting is superior to that of the previous CCFL system that LCDs used to provide brightness. LEDs can function in an "on/off" manner that increases the contrast overall: both whites and black benefitting. The practical application of this comes when watching a dark scene in a movie: instead of seeing detail in dark and shadow areas, everything gets lost amidst in a miasma that's more grey than black. If you compare CCFL to what a Plasma TV can display when it comes to black — well there is no comparison! The Plasma is better and always has been where CCFL is concerned.
LED backlighting doesn't have this problem and is able to provide the detail in black areas that help define a great picture. Especially when the resolution of that picture is being pumped up to 4K, as in one of the new 4K Ultra HD TVs.
The placement of the LEDs themselves differ from one company to the next also: for example, some TVs use what is called an "Edge" LED system. This is where the LEDs are arrayed at the side of the TV screen (hence, edge) and "throw" their light out towards the center. The other kind of LED lighting scheme is called Direct LED or more often now, "Full-Array" and consists of rows of LEDs placed at the back of the screen. Both of these types of LED systems can activate what is called "local dimming." This is where individual or groups of LEDs can be made to go "on/off" in specific screen areas; allowing greater control over specified areas of what is playing on the screen at any given time (for example, that great scene in Ghostbusters where the guys are huddling on the darkened rooftop as the Stay Puft Marshmallow Man lumbers by below).
Looking for "local dimming" in the text or specs that the flat screen TV has might turn up another name — that of "micro dimming." But it's still the same thing, and something that improves immensely upon the picture and comes thanks to LEDs.
Another aspect of LEDs are the colors it brings to the screen. Unlike CCFL, LEDs provide a much richer and "pure" color that more closely approximates our reality. Combine that with a high-resolution picture and it's no joke to say that the picture could look better than what you'd see for yourself — that is, if you were able to hold still as cars came at you in a chase scene, or not be fighting for oxygen while floating above the Earth in space.
A final point is that LED TVs possess a wider viewing angle. That might not be so important if you're the only one watching, but add a few friends and relatives to the couch when the big game has come on, and you've found yourself sitting at the edge or having to stand because you had to bring the snacks in, and suddenly it becomes really important. Good thing that this is true even of the smaller TVs, not just the big screen 55", 65" and larger ones.
There's a lot of features that make for a good TV, such as it being "smart," having wireless connectivity, great sound and able to display web pages like it was a computer. But it has to have a great picture and thank goodness today's flat screen TVs do.