In all these TVs, both LCD and 'LED', the picture is actually created by an LCD panel; the differences are in how that LCD panel is backlit.
Conventional LCD TVs have long used tubes to backlight the panel; an array of strip-lights. Trouble is, these use quite a lot of power, and can only be dimmed en masse: in other words, the back of the LCD is illuminated all the time the TV is on, even when it's showing an image which is all black. Were you to want to watch such a thing.
LED edge-lit TVs use an array of LED light-sources around the edge of the LCD panel, these shining into some kind of reflector/diffuser arrangement to give even lighting across the screen. The main advantages here are lower power consumption and the ability to make the TV slimmer.
Full LED TVs use more LED lightsources, arranged in a matrix behind the screen. They still have the benefits of edge-lit designs, LEDs taking up much less space than tubes.
If there are sufficient LEDs they open up the possibility of local dimming, in which a processor driven by the same video signal as that controlling the LCD panel can adjust the brightness of individual LEDs, or clusters of LEDs. This enables those LEDs lighting areas of the LCD panel showing a dark image to be dimmed right down, enhancing contrast.
RGB LED TVs - the name varies from brand to brand - will light sections of the LCD display panels not with the white LEDs used in the types mentioned above, but clusters of red, blue and green LEDs. These can be controlled to alter the colour of the backlighting in specific areas of the screen and this, combined with local dimming, theoretically improves colour fidelity as well as contrast.
It's important to reiterate that there's actually no such thing as an LED TV - well, not on the domestic market - and that all these 'LED TVs' don't light each pixel of the screen with its own LED, unlike those huge TV screens you see at concerts and sports events.
Those Jumbotrons/DiamondVisions and so ons really do create the picture with a matrix of lights, or rather a matrix of clusters of lights to create the colours, and rely on you being far enough away from them not to 'see the joins'.
As yet this technology is impractical in the much smaller screens we use at home, as is mapping each picture element, or pixel, of the screen to its own LED backlight. After all, in a typical 42in screen, you'd need just over two million lightsources, while for an RGB LED backlit system you'd need three times as many. And they'd have to be microscopically small.
Then again, if you could one day make a matrix of LEDs that small, and enable each and every one to be dimmed individually and adjusted in threes to provide any colour required, then you really would have a true LED TV...
That's what an OLED (organic LED) TV is: self-illuminating, with each picture element, or pixel, lit according to the subject-matter being shown. It has no backlight.
Sony sells an 11in version, but it's fiendishly expensive, and while others have shown larger prototypes, they're reportedly struggling to make these a viable commercial proposition.