LCD television uses liquid crystals that turn on and off to build an image across the screen. The crystals don't emit any light themselves so they need a light source behind the LCD panel. The light source has been fluorescent until recently, but newer models are introducing LED lighting as an alternative to fluorescent.
A true LED television uses the LEDs to generate the picture directly. For each pixel, a red, a green and a blue LED are needed. Unlike an LCD television, each LED generates its own illumination so there is no additional light source needed.