LED TVs.....
LED TVs are simply LCD TVs that use a different type of lamp made from light emitting diodes, in place of a fluorescent lamp (typically called CCFLs) for illuminating the picture. LCDs are screens of colored pixels. They do not create light, and as such, they require a light source. (Plasmas and the old style CRTs don't need lamps, because the phosphors that make up their screens emit light and color when zapped with electricity.)
LCD TVs....
With LCDs, the CCFL lamps are always placed directly behind the panel, which adds to the thickness. LED lamps are placed on around the panel beneath the screen bezel (2012 ES Samsung, LG and Toshiba LED models and many other brands) or like CCFLs, behind the LCD panel (select LGs, Sonys, Vizios, Sharps and EH Series Samsungs).
OLED
Meanwhile, OLED really is a whole new large-screen technology. The flat panel is made up of millions of tiny LEDs. The ?O? in OLED stands for ?organic? which means there is carbon within the molecules of the emissive (light producing) layer of the panel. Large-screen OLED panels need no lamps -- they are self illuminating. OLED HDTVs can be thinner and lighter than the skinniest LED LCDs, and have several other advantages over LCD TVs, regardless of whether the LCD is lit by LED or CCFL.
For instance, they provide very wide and consistent color no matter where you are seated in the room. LED LCDs tend to get significantly dimmer as you move away from the center, and many exhibit color shift. (There is one exception that I have found, the new WT50 Panasonic, which I reviewed here on HD Guru.)
The greatest attribute of OLED is the ability to have the deepest blacks of any flat panel technology. Unlike LED backlighting, which at best can only dim the LCD image in regions, OLEDs can produce a very low luminescence level down the individual pixel. This ability coupled with bright whites is why OLEDs are expected to have the highest contrast. OLEDs are very fast devices, changing intensity faster the best plasmas and the fastest (240 Hz) LED LCDs. This means there's no risk of motion blur.
OLEDs can make more colors than CCFL or LED panels. While impressive, this may not translate to a significant asset, since HDTV itself is limited to a specific color palette, one that a number of plasmas and LED HDTV already can meet or exceed.
How do AMOLED displays work?
OLED-Displays bests describes how AMOLED displays work:
Active matrix (AM) OLED displays stack cathode, organic, and anode layers on top of another layer ? or substrate ? that contains circuitry. The pixels are defined by the deposition of the organic material in a continuous, discrete ?dot? pattern. Each pixel is activated directly: A corresponding circuit delivers voltage to the cathode and anode materials, stimulating the middle organic layer. AMOLED pixels turn on and off more than three times faster than the speed of conventional motion picture film ? making these displays ideal for fluid, full-motion video.