THE LED ... LIGHT EMITTING DIODE TV
Behind the TV screen are LED lights as shown above. The LCD liquid crystal display panel creates the picture. For a white cloud, the LCD pixels lets all the light (from the LED backlights) through. For a black object, the LCD pixels become opaque and stop all the light. Colors are produed by controlling the amount of light coming through the red, green and blue pixels. In basic, lower price LED TV models all the LEDs put out a constant amount of light.
A FAP Full Array Panel, has more LEDs and the processor controls the amount of light the LEDs put out in zones. This is called Local Dimming. The more LEDs, the more zones, the brighter and higher contrast picture.
THE OLED ... ORGANIC LIGHT EMITTING DIODE TV
The OLED works 180 degrees opposite the LED. Each of the 8.2 million pixels (in a 4K TV) is a tiny light bulb. Each can be controlled indepently of the others. Turn off the light bulb you get black. Dead black.
Each red, green and blue pixels is controlled to produce the colors.
OLED panels are more expensive. Since each of the pixels or light bulbs are so small the amount of light they can produce is limited. OLED TVs are not as bright.
Many pundits consider the Sony QD-OLED the best TV you can buy
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