They are both technically LCD screens, the only difference is the backlight.
LCD TVs use one or two fluorescent tubes and a diffusing plate to light the whole screen from behind.
LED TVs use LEDs to light the screen from behind, depending on the TV the way the lighting is done can be done either by a couple of rows of leds along the edges of the screen a a similar diffusing panel to that used in LCD TVs, or the better ones use an array of LEDs directly behind the screen which are either constantly on, or in the really good screen can be dimmed individually to give better blacks and contrast.